Volume II Part 9 (1/2)
Do their walls lengthen laterally when they are empty, or does the 'matiere' inside stuff them more out, (itself increased from what sources?) when they are full? In either case, during this change from circle to hexagon, is the marrow getting thicker without getting longer? If so, the change in the angle of the cells is intentional, and probably is so; but the number of cells should have been the same: and further, the term 'hexagonal' can only be applied to the _section_ of a tubular cell, as in honeycomb, so that the floor and ceiling of our pith cell are left undescribed.
11. Having got thus much of (partly conjectural) idea of the mechanical structure of marrow, here follows the solitary vital, or mortal, fact in the whole business, given in one crus.h.i.+ng sentence at the close:---
”The medullary tissue” (first time of using this fine phrase for the marrow,--why can't he say marrowy tissue--'tissue moelleuse'?) ”appears very early struck with atony,” ('atonic,' want of tone,) ”above all, in its central parts.” And so ends all he has to say for the present about the marrow! and it never appears to occur to him for a moment, that if indeed the n.o.blest trees live all their lives in a state of healthy and robust paralysis, it is a distinction, hitherto unheard of, between vegetables and animals!
12. Two pages farther on, however, (p. 45,) we get more about the marrow, and of great interest,--to this effect, for I must abstract and complete here, instead of translating.
”The marrow itself is surrounded, as the centre of an electric cable is, by its guarding threads--that is to say, by a number of cords or threads coming between it and the wood, and differing from all others in the tree.
”The entire protecting cylinder composed of them has been called the 'etui,' (or needle-case,) of the marrow. But each of the cords which together form this etui, is itself composed of an almost infinitely delicate thread twisted into a screw, like the common spring of a letter-weigher or a Jack-in-the-box, but of exquisite fineness.” Upon this, two pages and an elaborate figure are given to these 'trachees'--tracheas, the French call them,--and we are never told the measure of them, either in diameter or length,[39] and still less, the use of them!
I collect, however, in my thoughts, what I have learned thus far.
13. A tree stem, it seems, is a growing thing, cracked outside, because its skin won't stretch, paralysed inside, because its marrow won't grow, but which continues the process of its life somehow, by knitted nerves without any nervous energy in them, protected by spiral springs without any spring in them.
Stay--I am going too fast. That coiling is perhaps prepared for some kind of uncoiling; and I will try if I can't learn something about it from some other book--noticing, as I pause to think where to look, the advantage of our English tongue in its pithy Saxon word, 'pith,' separating all our ideas of vegetable structure clearly from animal; while the poor Latin and French must use the entirely inaccurate words 'medulla' and 'moelle'; all, however, concurring in their recognition of a vital power of some essential kind in this white cord of cells: ”Medulla, sive illa vitalis anima est, ante se tendit, longitudinem impellens.” (Pliny, 'Of the Vine,' liber X., cap. xxi.) 'Vitalis anima'--yes--_that_ I accept; but 'longitudinem impellens,' I pause at; being not at all clear, yet, myself, about any impulsive power in the pith.[40]
14. However, I take up first, and with best hope, Dr. Asa Gray, who tells me (Art. 211) that pith consists of parenchyma, 'which is at first gorged with sap,' but that many stems expand so rapidly that their pith is torn into a mere lining or into horizontal plates; and that as the stem grows older, the pith becomes dry and light, and is 'then of no farther use to the plant.' But of what use it ever was, we are not informed; and the Doctor makes us his bow, so far as the professed article on pith goes; but, farther on, I find in his account of 'Sap-wood,' (Art. 224.) that in the germinating plantlet, the sap 'ascends first through the parenchyma, especially through its central portion or pith.' Whereby we are led back to our old question, what sap is, and where it comes from, with the now superadded question, whether the young pith is a mere succulent sponge, or an active power, and constructive mechanism, nourished by the abundant sap: as Columella has it,--
”Naturali enim spiritu omne alimentum virentis quasi quaedam anima, per _medullam_ trunci veluti per siphonem, trahitur in summum.”[41]
As none of these authors make any mention of a _communication_ between the cells of the pith, I conclude that the sap they are filled with is taken up by them, and used to construct their own thickening tissue.
15. Next, I take Balfour's 'Structural Botany,' and by his index, under the word 'Pith,' am referred to his articles 8, 72, and 75. In article 8, neither the word pith, nor any expression alluding to it, occurs.
In article 72, the stem of an outlaid tree is defined as consisting of 'pith, fibro-vascular and [42] woody tissue, medullary rays, bark, and epidermis.'
A more detailed statement follows, ill.u.s.trated by a figure surrounded by twenty-three letters--namely, two _b_ s, three _c_ s, four _e_ s, three _f_ s, one _l_, four _m_ s, three _p_ s, one _r_, and two _v_ s.
Eighteen or twenty minute sputters of dots may, with a good lens, be discerned to proceed from this alphabet, and to stop at various points, or lose themselves in the texture, of the represented wood. And, knowing now something of the matter beforehand, guessing a little more, and gleaning the rest with my finest gla.s.s, I achieve the elucidation of the figure, to the following extent, explicable without letters at all, by my more simple drawing, Figure 25.
16. (1) The inner circle full of little cells, diminis.h.i.+ng in size towards the outside, represents the pith, 'very large at this period of the growth'--(the first year, we are told in next page,) and 'very large'--he means in proportion to the rest of the branch. _How_ large he does not say, in his text, but states, in his note, that the figure is magnified 26 diameters. I have drawn mine by the more convenient multiplier of 30, and given the real size at B, _according to Balfour_:--but without believing him to be right. I never saw a maple stem of the first year so small.
[Ill.u.s.tration: FIG. 25.]
(2) The black band with white dots round the marrow, represents the marrow-sheath.
(3) From the marrow-sheath run the marrow-rays 'dividing the vascular circle into numerous compact segments.' A 'ray' cannot divide anything into a segment. Only a part.i.tion, or a knife, can do that. But we shall find presently that marrow _rays_ ought to be called marrow-_plates_, and are really mural, forming more or less continuous part.i.tions.
(4) The compact segments 'consist of woody vessels and of porous vessels.'
This is the first we have heard of woody _vessels_! He means the '_fibres_ ligneux' of Figuier; and represents them in each compartment, as at C (Fig.
25). without telling us why he draws the woody vessels as radiating. They appear to radiate, indeed, when wood is sawn across, but they are really upright.
(5) A moist layer of greenish cellular tissue called the cambium layer--black in Figure 25--and he draws it in flat arches, without saying why.
(6), (7), (8) Three layers of bark (called in his note Endophloeum; Mesophloeum, and Epiphloeum!) with 'laticiferous vessels.' [43]
(9) Epidermis. The three layers of bark being separated by single lines, I indicate the epidermis by a double one, with a rough fringe outside, and thus we have the parts of the section clearly visible and distinct for discussion, so far as this first figure goes,--without wanting one letter of all his three and twenty!
17. But on the next page, this ingenious author gives us a new figure, which professes to represent the same order of things in a longitudinal section; and in retracing that order sideways, instead of looking down, he not only introduces new terms, but misses one of his old layers in doing so,--thus: