Part 3 (1/2)

Figure 28 Finally, let us examine the family tree of a drone, or male bee. Eggs of worker bees that are not fertilized develop into drones. Hence, a drone has no ”father” and only a ”mother.” The queen's eggs, on the other hand, are fertilized by drones and develop into females (either workers or queens). A female bee has therefore both a ”mother” and a ”father.” Consequently, one drone has one parent (its mother), two grandparents (its mother's parents), three great-grandparents (two parents of its grand mother and one of its grandfather), five great-great-grandparents (two for each great-grandmother and one for its great-grandfather), and so on. The numbers in the family tree, 1, 1, 2, 3, 5..., form a Fibonacci se quence. The tree is presented graphically in Figure 29 Figure 29.

This all looks very intriguing-the same series of numbers applies to rabbits, to op tics, to stair climbing, and to drone family trees-but how is the Fibonacci sequence related to the Golden Ratio?

Figure 29 GOLDEN FIBONACCIS.

Examine again the Fibonacci sequence; 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987,..., and this time let us look at the ratios of successive numbers (calculated here to the sixth decimal place):

Do you recognize this last ratio? As we go farther and farther down the Fibonacci sequence, the ratio of two successive Fibonacci numbers oscillates about (being alternately greater or smaller) but comes closer and closer to the Golden Ratio. If we denote the n nth Fibonacci number by Fibonacci number by F Fn, and the next one by F Fn +1 +1, then we discovered that the ratio F Fn + + /F /Fn approaches as approaches as n n becomes larger. This property was discovered in 1611 (although possibly even earlier by an anonymous Italian) by the famous German astronomer Johannes Kepler, but more than a hundred years pa.s.sed before the relation between Fibonacci numbers and the Golden Ratio was proven (and even then not fully) by the Scottish mathematician Robert Simson (16871768). Kepler, by the way, apparently hit upon the Fibonacci sequence on his own and not via reading the becomes larger. This property was discovered in 1611 (although possibly even earlier by an anonymous Italian) by the famous German astronomer Johannes Kepler, but more than a hundred years pa.s.sed before the relation between Fibonacci numbers and the Golden Ratio was proven (and even then not fully) by the Scottish mathematician Robert Simson (16871768). Kepler, by the way, apparently hit upon the Fibonacci sequence on his own and not via reading the Liber abaci. Liber abaci.

But why should the terms in a sequence derived from the breeding of rabbits approach a ratio defined through the division of a line? To understand this connection, we have to go back to the astonis.h.i.+ng continued fraction we encountered in Chapter 4. Recall that we found that the Golden Ratio can be written as

In principle, we could calculate the value of by a series of successive approximations, in which we would interrupt the continued fraction farther and farther down. Suppose we attempted to do just that. We would find the series of values (reminder: 1 over a/b a/b is equal to is equal to b/a): b/a):

In other words, the successive approximations we find for the Golden Ratio are precisely equal to the ratios of Fibonacci numbers. No wonder then that as we go to higher and higher terms in the sequence the ratio converges to the Golden Ratio. This property is described beautifully in the book On Growth and Form On Growth and Form by the famous naturalist Sir D'Arcy Wentworth Thompson (18601948). He writes about the Fibonacci numbers: ”Of these famous and fascinating numbers a mathematical friend writes to me: 'All the romance of continued fractions, linear recurrence relations,... lies in them, and they are a source of endless curiosity. How interesting it is to see them striving to attain the unattainable, the Golden Ratio, for instance; and this is only one of hundreds of such relations.' ” The convergence to the Golden Ratio, by the way, explains the magic trick I described in Chapter 4. If you define a series of numbers by the property that each term (starting with the third) is equal to the sum of the two preceding ones, then irrespective of the two numbers you started with, as long as you go sufficiently far down the sequence, the ratio of two successive terms always approaches the Golden Ratio. by the famous naturalist Sir D'Arcy Wentworth Thompson (18601948). He writes about the Fibonacci numbers: ”Of these famous and fascinating numbers a mathematical friend writes to me: 'All the romance of continued fractions, linear recurrence relations,... lies in them, and they are a source of endless curiosity. How interesting it is to see them striving to attain the unattainable, the Golden Ratio, for instance; and this is only one of hundreds of such relations.' ” The convergence to the Golden Ratio, by the way, explains the magic trick I described in Chapter 4. If you define a series of numbers by the property that each term (starting with the third) is equal to the sum of the two preceding ones, then irrespective of the two numbers you started with, as long as you go sufficiently far down the sequence, the ratio of two successive terms always approaches the Golden Ratio.

The Fibonacci numbers, like the ”aspiration” of their ratios-the Golden Ratio-have some truly amazing properties. The list of mathematical relations involving Fibonacci numbers is literally endless. Here are just a handful of them.

”Squaring” Rectangles If you sum up an odd number of products of successive Fibonacci numbers, like the three products 11+12+23, then the sum (1+2+6 = 9) is equal to the square of the last Fibonacci number you used in the products (in this case, 3 32 = = 9). To take another example, if we sum up seven products, 11+ 12+23+35+5 8+ 8 13+ 13 21 = 441, the sum (441) is equal to the square of the last number used (21 9). To take another example, if we sum up seven products, 11+ 12+23+35+5 8+ 8 13+ 13 21 = 441, the sum (441) is equal to the square of the last number used (212 = 441). Similarly, summing up the eleven products 11+ 1 2+23+35+58+ 813+ 1321+21 34+ 3455+ 5589+ 89 144 = 144 = 441). Similarly, summing up the eleven products 11+ 1 2+23+35+58+ 813+ 1321+21 34+ 3455+ 5589+ 89 144 = 1442. This property can be represented beautifully by a figure (Figure 30). Any odd number of rectangles with sides equal to successive Fibonacci numbers fits precisely into a square. The figure shows an example with seven such rectangles.

Figure 30 Eleven Is the Sin In the drama The Piccohmini The Piccohmini by the German playwright and poet Friedrich Schiller, astrologer Seni declares: ”Elf ist die Sunde. Elfe uber-schreiten die zehn Gebote” (”Eleven is the sin. Eleven transgresses the Ten Commandments”), expressing an opinion that dates back to medieval times. The Fibonacci sequence, on the other hand, has a property related to the number 11, which, far from being sinful, is quite beautiful. by the German playwright and poet Friedrich Schiller, astrologer Seni declares: ”Elf ist die Sunde. Elfe uber-schreiten die zehn Gebote” (”Eleven is the sin. Eleven transgresses the Ten Commandments”), expressing an opinion that dates back to medieval times. The Fibonacci sequence, on the other hand, has a property related to the number 11, which, far from being sinful, is quite beautiful.

Suppose we sum up the first ten consecutive Fibonacci numbers: 1+ 1+2+ 3+ 5+8+ 13+ 21+ 34+ 55 = 143. This sum is divisible evenly by 11 (143/11 = 13). The same is true for the sum of any any ten consecutive Fibonacci numbers. For example, 55+ 89+ 144+ 233+ 377+ 610+ 987+ 1,597+ 2,584+ 4,181 = 10,857, and 10,857 is divisible by 11, 10,857/11 = 987. If you examine these two examples, you discover something else. The sum of any ten consecutive numbers is always equal to 11 times the seventh number. You can use this property to amaze an audience by the speed with which you can add any ten successive Fibonacci numbers. ten consecutive Fibonacci numbers. For example, 55+ 89+ 144+ 233+ 377+ 610+ 987+ 1,597+ 2,584+ 4,181 = 10,857, and 10,857 is divisible by 11, 10,857/11 = 987. If you examine these two examples, you discover something else. The sum of any ten consecutive numbers is always equal to 11 times the seventh number. You can use this property to amaze an audience by the speed with which you can add any ten successive Fibonacci numbers.

Revenge of the s.e.xagesimal?

As you recall, for reasons that are not entirely clear, the ancient Babylonians used base 60 (the s.e.xagesimal base) in their counting system. Although not related to the Babylonian number system, the number 60 happens to play a role in the Fibonacci sequence.

Fibonacci numbers become very large quite rapidly, because you always add two successive Fibonacci numbers to find the next one. In fact, we are quite lucky that rabbits don't live forever, or we would all be inundated with rabbits. While the fifth Fibonacci number is only 5, the 125th is already 59,425,114,757,512,643,212,875,125. Interestingly, the unit digit repeats itself with a periodicity of 60 (namely, after every 60 numbers). For example, the second number is 1, the sixty-second number is 4,052,739,537,881 (also ending in 1); the 122 is already 59,425,114,757,512,643,212,875,125. Interestingly, the unit digit repeats itself with a periodicity of 60 (namely, after every 60 numbers). For example, the second number is 1, the sixty-second number is 4,052,739,537,881 (also ending in 1); the 122nd number, 14,028,366,653,498,915,298,923,761, also ends in 1; and so does the 182 number, 14,028,366,653,498,915,298,923,761, also ends in 1; and so does the 182nd; and so on. Similarly, the fourteenth number is 377; the seventy-fourth number (sixty numbers farther along the sequence) 1,304,969,544,928,657 also ends in 7; and so on. This property was discovered in 1774 by the Italian-born French mathematician Joseph Louis Lagrange (17361813), who is responsible for many works in number theory and mechanics and who also studied the stability of the solar system. The last two digits (e.g., 01, 01, 02, 03, 05, 08, 13, 21...) repeat in the sequence with a periodicity of 300 and the last three digits repeat with a periodicity of 1,500. In 1963 Stephen P. Geller used an IBM 1620 computer to show that the last four digits repeat every 15,000 times, the last five repeat every 150,000 times, and finally, after the computer ran for nearly three hours, a repet.i.tion of the last six digits appeared at the 1,500,000th Fibonacci number. Being unaware of the fact that a general theorem concerning the periodicity of the last digits could be proven, Geller commented: ”There does not yet seem to be any way of guessing the next period, but perhaps a new program for the machine which will permit initialization at any point in the sequence for a test will cut down computer time enough so that more data can be gathered.” Shortly thereafter, however, Israeli mathematician Dov Jarden pointed out that one can prove rigorously that for any number of last digits from three and up, the periodicity is simply fifteen times ten to a power that is one less than the number of digits (e.g., for seven digits it is 15 10, or 15 million). Fibonacci number. Being unaware of the fact that a general theorem concerning the periodicity of the last digits could be proven, Geller commented: ”There does not yet seem to be any way of guessing the next period, but perhaps a new program for the machine which will permit initialization at any point in the sequence for a test will cut down computer time enough so that more data can be gathered.” Shortly thereafter, however, Israeli mathematician Dov Jarden pointed out that one can prove rigorously that for any number of last digits from three and up, the periodicity is simply fifteen times ten to a power that is one less than the number of digits (e.g., for seven digits it is 15 10, or 15 million).

Why ? ?

The properties of our universe, from the sizes of atoms to the sizes of galaxies, are determined by the values of a few numbers known as constants of nature. These constants include a measure of the strengths of all the basic forces-gravitational, electromagnetic, and two nuclear forces. The strength of the familiar electromagnetic force between two electrons, for example, is expressed in physics in terms of a constant known as the fine structure constant. The value of this constant, almost exactly , has puzzled many generations of physicists. A joke made about the famous English physicist Paul Dirac (19021984), one of the founders of quantum mechanics, says that upon arrival to heaven he was allowed to ask G.o.d one question. His question was: ”Why , has puzzled many generations of physicists. A joke made about the famous English physicist Paul Dirac (19021984), one of the founders of quantum mechanics, says that upon arrival to heaven he was allowed to ask G.o.d one question. His question was: ”Why The Fibonacci sequence also contains one absolutely remarkable number-its eleventh number, 89. The value of in decimal representation is equal to: 0.01123595... Suppose you arrange the Fibonacci numbers 1, 1, 2, 3, 5, 8, 13, 21,...as decimal fractions in the following way: in decimal representation is equal to: 0.01123595... Suppose you arrange the Fibonacci numbers 1, 1, 2, 3, 5, 8, 13, 21,...as decimal fractions in the following way:

In other words, the units digit in the first Fibonacci number is in the second decimal place, that of the second is in the third decimal place, and so on (the units digit of the n nth Fibonacci number is in the Fibonacci number is in the (n (n+1)th decimal place). Now add all of those numbers up. In the preceding list we would obtain 0.01123595..., which is equal to decimal place). Now add all of those numbers up. In the preceding list we would obtain 0.01123595..., which is equal to . .

Lightning Addition Trick Some people can add numbers very quickly in their heads. The Fibonacci sequence allows a person to perform such lightning addition tricks without much effort. The sum of all the Fibonacci numbers from the first to the n nth is simply equal to the is simply equal to the (n (n+ 2)th number minus 1. For example, the sum of the first ten numbers, 1+ 1+2+ 3+ 5+ 8+ 13+21+34+ 55 = 143, is equal to the twelfth number (144) minus 1. The sum of the first seventy-eight numbers is equal to the eightieth number minus 1; and so on. Therefore, you can have someone write a long column of numbers starting with 1, 1, and continuing using the definition of the Fibonacci sequence (that each new number be the sum of the two previous ones). Tell this person to draw a line between some arbitrary two numbers in the column and you will be able, at a glance, to give the sum of all the numbers prior to the line. That sum will be equal to the second term after the line minus one. number minus 1. For example, the sum of the first ten numbers, 1+ 1+2+ 3+ 5+ 8+ 13+21+34+ 55 = 143, is equal to the twelfth number (144) minus 1. The sum of the first seventy-eight numbers is equal to the eightieth number minus 1; and so on. Therefore, you can have someone write a long column of numbers starting with 1, 1, and continuing using the definition of the Fibonacci sequence (that each new number be the sum of the two previous ones). Tell this person to draw a line between some arbitrary two numbers in the column and you will be able, at a glance, to give the sum of all the numbers prior to the line. That sum will be equal to the second term after the line minus one.

Pythagorean Fibonaccis Oddly enough, Fibonacci numbers can even be related to Pythagorean triples. The latter, as you recall, are triples of numbers that can serve as the lengths of the sides of a right-angled triangle (like the numbers 3, 4, 5). Take any four consecutive Fibonacci numbers, such as 1, 2, 3, 5. The product of the outer numbers, 15 = 5, twice the product of the inner terms, 223 = 12, and the sum of the squares of the inner terms, 22+32 = 13, give the three legs in the Pythagorean triple, 5,12,13(5 = 13, give the three legs in the Pythagorean triple, 5,12,13(52+122=132). But this is not all. Notice also that the third number, 13, is itself a Fibonacci number. This property was discovered by the mathematician Charles Raine.

Given the numerous wonders that the Fibonacci numbers hold in store (we shall soon encounter many more), it should come as no surprise that mathematicians looked for some efficient method for calculating these numbers, F Fn' for any value of for any value of n n While in principle this is not a problem, since if we need the 100 While in principle this is not a problem, since if we need the 100th number we simply have to add up the 98 number we simply have to add up the 98th and the 99 and the 99th numbers, this still means that we first need to calculate all the numbers up to the 99 numbers, this still means that we first need to calculate all the numbers up to the 99th, which can be quite tedious. As the late comedian George Burns (in his book How to Live to Be 100 or More) How to Live to Be 100 or More) once put it: ”How do you live to be 100 or more? There are certain things you have to do. The most important one is you have to be sure to make it to 99.” once put it: ”How do you live to be 100 or more? There are certain things you have to do. The most important one is you have to be sure to make it to 99.”

In the middle of the nineteenth century, the French mathematician Jacques Phillipe Marie Binet (17861856) rediscovered a formula that was apparently known already in the eighteenth century to the most prolific mathematician in history, Leonard Euler (17071783), and to the French mathematician Abraham de Moivre (16671754). The formula allows you to find the value of any Fibonacci number, F Fn, if its place in the sequence, n n, is known. This Binet formula relies entirely on the Golden Ratio

At first glance, this is a formidably disconcerting formula, since it is not even obvious that upon subst.i.tution of various values of n n it would produce whole numbers (which all the terms in the Fibonacci sequence are). Since we already know that the Fibonacci numbers are intimately related to the Golden Ratio, things start to look a little bit more rea.s.suring when we realize that the first term inside the brackets is, in fact, simply the Golden Ratio raised to the it would produce whole numbers (which all the terms in the Fibonacci sequence are). Since we already know that the Fibonacci numbers are intimately related to the Golden Ratio, things start to look a little bit more rea.s.suring when we realize that the first term inside the brackets is, in fact, simply the Golden Ratio raised to the n nth power, power, n n, and the second is (-1/ )n. (Recall from earlier that the negative solution of the quadratic equation defining is equal to -1/ .) Using a simple scientific pocket calculator you can test for a few values of n n that Binet's formula is indeed giving the Fibonacci numbers correctly. For relatively large values of that Binet's formula is indeed giving the Fibonacci numbers correctly. For relatively large values of n n, the second term in the brackets above becomes very small, and you can simply take F Fn to be the closest whole number to to be the closest whole number to . For example, for . For example, for n n=10, is equal to 55.0036, and the tenth Fibonacci number is 55. is equal to 55.0036, and the tenth Fibonacci number is 55.

Just as an amus.e.m.e.nt, you may wonder if there is a Fibonacci number with precisely 666 666 digits. Mathematician and author Clifford A. Pickover calls numbers a.s.sociated with digits. Mathematician and author Clifford A. Pickover calls numbers a.s.sociated with 666 666 ”apocalyptic.” He found that the 3,184 ”apocalyptic.” He found that the 3,184th Fibonacci number has Fibonacci number has 666 666 digits. digits.

Once discovered, Fibonacci numbers seemed to start popping up everywhere in nature. A few fascinating examples are provided by botany AS THE SUNFLOWER TURNS ON HER G.o.d.

The leaves along a twig of a plant or the stems along a branch tend to grow in positions that would optimize their exposure to sun, rain, and air. As a vertical stem grows, it produces leaves at quite regular s.p.a.cings. However, the leaves do not grow directly one above the other, because this would s.h.i.+eld the lower leaves from the moisture and sunlight they need. Rather, the pa.s.sage from one leaf to the next (or from one stem to the next along a branch) is characterized by a screw-type displacement around the stem (as in Figure 31 Figure 31). Similar arrangements of re peating units can be found in the scales of a pinecone or the seeds of a sunflower. This phenomenon is called phyllotaxis phyllotaxis (”leaf arrangement” in Greek), a word coined in 1754 by the Swiss naturalist Charles Bonnet (17201793). For example, in ba.s.swoods leaves occur generally on two opposite sides (corresponding to half a turn around the stem), which is known as a phyllotactic ratio. In other plants, such as the hazel, blackberry, and beech, pa.s.sing from one leaf to the next involves one-third of a turn ( phyllotactic ratio). Similarly, the apple, the coast live oak, and the apricot have leaves every of a turn, and the pear and the weeping willow have them every of a turn. (”leaf arrangement” in Greek), a word coined in 1754 by the Swiss naturalist Charles Bonnet (17201793). For example, in ba.s.swoods leaves occur generally on two opposite sides (corresponding to half a turn around the stem), which is known as a phyllotactic ratio. In other plants, such as the hazel, blackberry, and beech, pa.s.sing from one leaf to the next involves one-third of a turn ( phyllotactic ratio). Similarly, the apple, the coast live oak, and the apricot have leaves every of a turn, and the pear and the weeping willow have them every of a turn. Figure 31 Figure 31 ill.u.s.trates a case where it took three complete turns to pa.s.s through eight stems (a phyllotactic ratio of ). You'll notice that all the fractions that are observed are ratios of alternate members of the Fibonacci sequence. ill.u.s.trates a case where it took three complete turns to pa.s.s through eight stems (a phyllotactic ratio of ). You'll notice that all the fractions that are observed are ratios of alternate members of the Fibonacci sequence.

Figure 31 The fact that leaves of plants follow certain patterns was first noted in antiquity by Theophrastus (ca. 372 B.C. B.C.-ca. 287 B.C. B.C.) in Enquiry into Plants. Enquiry into Plants. He remarks: ”those that have flat leaves have them in a regular series.” Pliny the Elder He remarks: ”those that have flat leaves have them in a regular series.” Pliny the Elder (A.D. (A.D. 2379) made a similar observation in his monumental 2379) made a similar observation in his monumental Natural History Natural History, where he talks about ”regular intervals” between leaves ”arranged circularly around the branches.” The study of phyllotaxis did not go much beyond these early, qualitative observations until the fifteenth century, when Leonardo da Vinci (14521519) added a quant.i.tative element to the description of leaf arrangements by noting that the leaves were arranged in spiral patterns, with cycles of five (corresponding to an angle of of a turn). The first person to discover (intuitively) the relation between phyllotaxis and the Fibonacci numbers was the astronomer Johannes Kepler. Kepler wrote: ”It is in the likeness of this self-developing series [referring to the recursive property of the Fibonacci sequence] that the faculty of propagation is, in my opinion, formed; and so in a flower the authentic flag of this faculty is shown, the pentagon.”

Charles Bonnet initiated serious studies in observational phyllotaxis. In his 1754 book Recherches sur l'Usage des Feuilles dans les Plantes Recherches sur l'Usage des Feuilles dans les Plantes (Research on the use of leaves in plants) he gives a clear description of phyllotaxis. While working with the mathematician G. L. Calandrini, Bonnet may have also discovered that sets of spiral rows (now known as parastichies) appear in some plants, like the scales of a fir cone or a pineapple. (Research on the use of leaves in plants) he gives a clear description of phyllotaxis. While working with the mathematician G. L. Calandrini, Bonnet may have also discovered that sets of spiral rows (now known as parastichies) appear in some plants, like the scales of a fir cone or a pineapple.

The history of truly mathematical mathematical phyllotaxis (as opposed to the purely descriptive approaches) begins in the nineteenth century with the works of botanist Karl Friedric Schimper (published in 1830), his friend Alexander Braun (published in 1835), and the crystallographer Auguste Bravais and his botanist brother Louis (published in 1837). These researchers discovered the general rule that phyllotactic ratios could be expressed by ratios of terms of the Fibonacci series (like ; ) and also noted the appearance of consecutive Fibonacci numbers in the parastichies of pinecones and pineapples. phyllotaxis (as opposed to the purely descriptive approaches) begins in the nineteenth century with the works of botanist Karl Friedric Schimper (published in 1830), his friend Alexander Braun (published in 1835), and the crystallographer Auguste Bravais and his botanist brother Louis (published in 1837). These researchers discovered the general rule that phyllotactic ratios could be expressed by ratios of terms of the Fibonacci series (like ; ) and also noted the appearance of consecutive Fibonacci numbers in the parastichies of pinecones and pineapples.

Pineapples indeed provide a truly beautiful manifestation of a Fibonacci-based phyllotaxis (Figure 32). Each hexagonal scale on the surface of the pineapple is a part of three different spirals. In the figure you can see one of eight parallel rows sloping gently from lower left to upper right, one of thirteen parallel rows that slope more steeply from lower right to upper left, and one of twenty-one parallel rows that are very steep (from lower left to upper right). Most pineapples have five, eight, thirteen, or twenty-one spirals of increasing steepness on their surface. All of these are Fibonacci numbers.

Figure 32

Figure 33 How do plants know to arrange their leaves in these Fibonacci patterns? The growth of the plant takes place at the tip of the stem (called the meristem), which has a conical shape (being thinnest at the tip). Leaves that are farther down from the tip (namely, which grew earlier) tend to be radially farther out from the stem's center when viewed from the top (because the stem is thicker there). Figure 33 Figure 33 shows such a view of the stem from the top, where the leaves are numbered according to their order of appearance. The leaf numbered 0, which appeared first, is by now the farthest down from the meristem and the farthest out from the stem's center. Botanist A. H. Church in his 1904 book shows such a view of the stem from the top, where the leaves are numbered according to their order of appearance. The leaf numbered 0, which appeared first, is by now the farthest down from the meristem and the farthest out from the stem's center. Botanist A. H. Church in his 1904 book On the Relation of Phyllotaxis to Mechanical Laws On the Relation of Phyllotaxis to Mechanical Laws first emphasized the importance of this type of representation for the understanding of phyllotaxis. What we find (by imagining a curve that connects leaves 0 to 5 in first emphasized the importance of this type of representation for the understanding of phyllotaxis. What we find (by imagining a curve that connects leaves 0 to 5 in Figure 33 Figure 33) is that successive leaves sit along a tightly wound spiral, known as the generative spiral. The important quant.i.ty that characterizes the location of the leaves is the angle between the lines connecting the stem's center with successive leaves. One of the discoveries of the Bravais brothers in 1837 was that new leaves advance roughly by the same angle around the circle and that this angle (known as the divergence angle) is usually close to 137.5 . Are you shocked to hear that this value is determined by the Golden Ratio? The angle that divides a complete turn in a Golden Ratio is 360/ = 222.5 . Since this is more than half a circle (180 degree), we should measure it going in the opposite direction around the circle. In other words, we should subtract 222.5 from 360, giving us the observed angle of 137.5 degree (sometimes called the Golden Angle).

In a pioneering work in 1907, German mathematician G. van Iterson showed that if you closely pack successive points separated by 137.5 degree on tightly wound spirals, then the eye would pick out one family of spiral patterns winding clockwise and one counterclockwise. The numbers of spirals in the two families tend to be consecutive Fibonacci numbers, since the ratio of such numbers approaches the Golden Ratio.

Such counterwinding spirals are most spectacularly exhibited by the arrangement of the florets in sunflowers. When you look on the head of a sunflower (Figure 34), you will notice both clockwise and counterclockwise spiral patterns formed by the florets. Clearly the florets grow in a way that affords the most efficient sharing of horizontal s.p.a.ce. The numbers of these spirals usually depend on the size of the sunflower. Most commonly there are thirty-four spirals going one way and fifty-five the other, but sunflowers with ratios of numbers of spirals of 89/55, 144/89, and even (at least one; reported by a Vermont couple to the Scientific American Scientific American in 1951) 233/144 have been seen. All of these are, of course, ratios of adjacent Fibonacci numbers. In the largest sunflowers, the structure stretches from one pair of consecutive Fibonacci numbers to the next higher, when we move from the center to the periphery. in 1951) 233/144 have been seen. All of these are, of course, ratios of adjacent Fibonacci numbers. In the largest sunflowers, the structure stretches from one pair of consecutive Fibonacci numbers to the next higher, when we move from the center to the periphery.

Figure 34 The petal counts and petal arrangements of some flowers also harbor Fibonacci numbers and Golden Ratio connections. Many people have relied (at least symbolically) at some point in their lives on the numbers of petals of daisies to satisfy their curiosity about the intriguing question: ”She loves me, she loves me not.” Most field daisies have thirteen, twenty-one, or thirty-four petals, all Fibonacci numbers. (Wouldn't it be nice to know in advance if the daisy has an even or odd number of petals?) The number of petals simply reflects the number of spirals in one family.

The beautifully symmetric arrangement of the petals of roses is also based on the Golden Ratio. If you dissect a rose (petal by petal), you will discover the positions of its tightly packed petals. Figure 35 Figure 35 presents a schematic in which the petals have been numbered. The angles defining the positions (in fractions of a full turn) of the petals are the fractional part of simple multiples of . Petal 1 is 0.618 (the fractional part of 1 presents a schematic in which the petals have been numbered. The angles defining the positions (in fractions of a full turn) of the petals are the fractional part of simple multiples of . Petal 1 is 0.618 (the fractional part of 1 ) of a turn from petal 0, petal 2 is 0.236 (the fractional part of 2 ) of a turn from petal 1, and so on.

Figure 35 This description shows that the 2,300-year-old puzzle of the origins of phyllotaxis reduces to the basic question: Why are successive leaves separated by the Golden Angle of 137.5 degree? The attempts to answer this question come in two flavors: theories that concentrate on the geometry of the configuration, and on simple mathematical rules that can generate this geometry; and models that suggest an actual dynamical cause for the observed behavior. Landmark works of the first type (e.g., by mathematicians Harold S. M. c.o.xeter and I. Adler and by crystallographer N. Rivier) show that buds which are placed along the generative spiral separated by the Golden Angle are close-packed most efficiently. This is easy to understand. If the divergence angle was, let's say, 120 degree (which is 360/3) or any other rational multiple of 360 degree, then the leaves would have aligned radially (along three lines in the case of 120 degree), leaving large s.p.a.ces in between. On the other hand, a divergence angle like the Golden Angle (which is an irrational multiple of 360 degree) ensures that buds do not line up along any specific radial direction and they fill the s.p.a.ces efficiently. The Golden Angle proves to be even better than other irrational multiples of 360 degree because the Golden Ratio is the most irrational of all irrational numbers in the following sense. Recall that the Golden Ratio is equal to a continued fraction composed entirely of 1s. That continued fraction converges more slowly than any other continued fraction. In other words, the Golden Ratio is farther away from being expressible as a fraction than any other irrational number.