Part 13 (2/2)

The sporangia are borne on special scale-like leaves, and arranged very much as in certain pteridophytes, notably the clubproduced near together, the two kinds are borne on special branches, or even on distinct trees (_eg_ red cedar) In the Scotch pine thetheated in s shoot ter 77, _A_ ?) The individual leaves (sporophylls) are nearly triangular in shape, and attached by the sia (pollen sacs) (_C_, _sp_), opening by a longitudinal slit, and filled with innumerable yellow microspores (pollen spores), which fall out as a shower of yellow dust if the branch is shaken

The ia (ovules) are borne on similar leaves, known as carpels, and, like the pollen sacs, borne in pairs, but on the upper side of the sporophyll instead of the lower The female flowers appear when the pollen is ripe The leaves of which they are composed are thicker than those of the male flowers, and of a pinkish color At the base on the upper side are borne the two ovules (h the centre is a ridge that ends in a little spine or point

The ovule-bearing leaf has on the back a scale with fringed edge (_F_, _sc_), quite conspicuous when the flower is young, but scarcely to be detected in the older cone Fro 75, _A_), but the process is a slow one, occupying two years Shortly after the pollen is shed, the feht, bend doard, and assu considerably in size for a short tirow for several months

[Illustration: FIG 75--Scotch pine (_Pinus sylvestris_) _A_, a ripe cone,_B_, a year-old cone, 1 _C_, longitudinal section of _B_ _D_, a single scale of _B_, showing the sporangia (ovules) (_o_), 2 _E_, a scale froitudinal section of a ripe seed, 3 _e seed, 2 _r_, the pri the first leaves of the young plant still surrounded by the endosper fro the circle of primary leaves (cotyledons), 1 _K_, section of the same, 2 _b_, the ter plant, 25 _fb_ a fibro-vascular bundle _M_, cross-section of the root, 25 _x_, wood _ph_ bast, of the fibro-vascular bundle]

In Figure 75, _B_, is shown such a flower as it appears in the winter and early spring following The leaves are thick and fleshy, closely pressed together, as is seen by dividing the flower lengthwise, and each leaf ends in a long point (_D_) The ovules are still very s, the flower (cone) increases rapidly in size and beco also very much in size If a scale from such a cone is examined about the first of June, the ovules will probably be nearly full-grown, oval, whitish bodies two to three itudinal section of the scale through the ovule will show the general structure Such a section is shown in Figure 77, _G_ Coia of the pteridophytes, the first difference that strikes us is the presence of an outer coat or integule e and does not lie free in the cavity of the sporangium, but is in close contact with its wall It is filled with a colorless tissue, the prothallium, and if mature, with care it is possible to see, even with a hand lens, two or onia, which here are very large The integument is not entirely closed at the top, but leaves a little opening through which the pollen spores entered when the floas first foronia are fertilized the outer parts of the ovule become hard and brown, and serve to protect the embryo plant, which reaches a considerable size before the sporangium falls off As the walls of the ovule harden, the carpel or leaf bearing it undergoes a si extremely hard and woody, and as each one ends in a sharp spine, and they are tightly packed together, it is al 75, _A_) re, about the time the flowers are e the ripened ovules, now called seeds Each seed (_E_, _s_) is surrounded by a membranous envelope derived from the scale to which it is attached, which beco of the cones is caused by drying, and if a nu, and allowed to dry in an ordinary room, they will in a day or two open, often with a sharp, crackling sound, and scatter the ripe seeds

A section of a ripe seed (_F_) shows the e tissue derived from the prothallium cells, and called the ”endosperm” This fills up the whole seed which is surrounded by the hardened shell derived froated with a circle of s of the ovule tohich is directed the root of the eed forits vitality, but if the proper conditions are provided, the erowth of the eood soil and kept moderately warm and moist At the end of a week or two some of the seeds will probably have sprouted The seed absorbs water, and the protoplas to feed upon the nourishi+ng substances in the cells of the endosperth, and the root pushes out of the seed growing rapidly doard and fastening itself in the soil (_G_, _r_)

Cutting the seed lengthe find that the leaves have increased reen (one of the few cases where chlorophyll is forht) As these leaves (called ”cotyledons” or seed leaves) increase in length, they gradually withdraw fro plant enters upon an independent existence

The young plant has a circle of leaves, about six in nu point of the sterows (Fig 75, _K_, _b_) A cross-section of the young steed in a circle (_S_, _fb_) The root shows a central fibro-vascular cylinder surrounded by a dark-colored ground tissue Growing fro 75, _M_)

For exa the microscopic structure of the pine, fresh material is for most purposes to be preferred, but alcoholic material will answer, and as the alcohol hardens the resin, it is for that reason preferable

Cross-sections of the leaf, when sufficiently nified, show that the outer colorless border of the section is coular cells with very thick outer walls, and irregular groups of cells lying below the silvery and clearer than the epider reduced to a single row, in others forreen tissue of the leaf is much more compact than in the fern we examined, and the cells are more nearly round and the intercellular spaces smaller The chloroplasts are nuh the green tissue are several resin passages (_r_), each surrounded by a circle of colorless, thick-walled cells, like those under the epider pores--(Fig 76, _J_), below each of which is an intercellular space (_i_) They are in structure like those of the ferns, but the walls of the guard cells are much thickened like the other epidermal cells

Each leaf is traversed by two fibro-vascular bundles of entirely different structure from those of the ferns Each is divided into two nearly equal parts, the wood (_x_) lying toward the inner, flat side of the leaf, the bast (_T_) toward the outer, convex side This type of bundle, called ”collateral,” is the common form found in the stems and leaves of seed plants The cells of the wood or xyleer than those of the bast or phloem, and have thicker walls than any of the phloem cells, except the outermost ones which are thick-walled fibres like those under the epidere colorless cells, and surrounding the whole central area is a single line of cells that separates it sharply froitudinal sections, the cells, except of the ated The mesophyll cells, however, are short and the intercellular spaces much more evident than in the cross-section The colorless cells have frequently rounded depressions or pits upon their walls, and in the fibro-vascular bundle the difference between the two portions becouished by the presence of vessels with close, spiral or ring-shaped thickenings, while in the phloem are found sieve tubes, not unlike those in the ferns

The fibro-vascular bundles of the ste plant show a structure quite similar to that of the leaf, but very soon a difference is manifested Between the two parts of the bundle the cells continue to divide and add constantly to the size of the bundle, and at the sarowing cells, so that very early we find a ring of growing cells extending co increase in nu to their rapid division, those on the borders of the ring lose the power of dividing, and gradually assu 76, _B_, _ca iscontinues co 76, _A_, _cam_) The spaces between the bundles do not increase materially in breadth, and as the bundles increase in size beco in older stems as mere lines between the solid masses of wood that make up the inner portion of the bundles These are the primary medullary rays, and connect the pith in the centre of the stem with the bark Later, sile cell thick, and appearing when seen in cross-section as a single row of elongated cells (_C_, _m_)

As the stem increases in diameter the bundles become broader and broader toward the outside, and taper to a point toward the centre, appearing wedge-shaped, the inner ends projecting into the pith The outer limits of the bundles are not nearly so distinct, and it is not easy to tell when the phloeins

A careful examination of a cross-section of the bark shows first, if taken from a branch not more than two or three years old, the epidermis composed of cells not unlike those of the leaf, but whose walls are usually browner Underneath are cells with brownish walls, and often ive the brown color to the bark, and later both epiderround tissue becoround tissue is e, loose cells, the outer ones containing a good deal of chlorophyll Here and there are large resin ducts (Fig 76, _H_), appearing in cross-section as oval openings surrounded by several concentric rows of cells, the innermost smaller and with denser contents These secrete the resin that fills the duct and oozes out when the stem is cut All of the cells of the bark contain nified, is seen to berows Their walls are not very thick and the cells are usually somewhat flattened in a radial direction

Soer than the others, and these are found to be, when exa 76, _E_) with numerous lateral sieve plates quite similar to those found in the stems of ferns

[Illustration: FIG 76--Scotch pine _A_, cross-section of a two-year-old branch, 3 _p_, pith _c_, bark The radiating lines are medullary rays _r_, resin ducts _B_, part of the same, 150

_cam_ cambium cells _x_, tracheids _C_, cross-section of a two-year-old branch at the point where the two growth rings join: _I_, the cells of the first year's growth; _II_, those of the second year

_itudinal section of a branch, showing the form of the tracheids and the bordered pits upon their walls _m_, medullary ray, 150 _E_, part of a sieve tube, 300

_F_, cross-section of a tracheid passing through two of the pits in the wall (_p_), 300 _G_, longitudinal section of a branch, at right angles to the h the wall of a tracheid, bearing a row of pits, 150 _H_, cross-section of a resin duct, 150 _I_, cross-section of a leaf, 20 _fb_ fibro-vascular bundle _r_, resin duct _J_, section of a breathing pore, 150 _i_, the air space below it]