Dec. 22, 1887] 



NA TURE 



185 



Confining our attention to one of the innermost, oldest 

 cells of the cambiuirj, which has ceased dividing {aa 

 in Fig. 5), we find that it enlarges somewhat in 

 the radial direction, and then its hitherto very thin 

 walls become thicker ; in fact, the protoplasm in its 

 interior absorbs food-materials, and changes them into 

 a peculiar substance which it plasters or builds on 

 to the inner sides of the cell- wall, so to speak, until the 

 wall is much thicker. This thickening process is with- 

 held at certain places only — the thin depressions already 

 referred to. Two chief changes result now : (i) the whole 

 of the living contents of the young wood-cell gradually 

 become used up, and eventually disappear without leaving 

 any trace ; and (2) the thickening substance built on 

 to the inside of the walls undergoes changes which convert 

 it into true wood-substance — in botanical language, the 

 walls become lignified. The cells b and c in Fig. 5 

 illustrate what is meant. 



During all these changes, which occupy several or even 

 many hours or days, according to circumstances, it will be 

 observed that the definitive shape of the cell is gradually 

 completed, and then alters very little : the prismatic 

 cambium-cell has become a prismatic tracheide, with 

 thicker, lignified walls, and containing air and water (with 

 minute quantities of mineral substances dissolved in it) 

 in place of protoplasm and nutritive substances. It is not 

 necessary here to speak of other and more subtle changes 

 which cause slight displacements, &c., of these cells. 



If I have succeeded in making the chief points in this 

 somewhat conrplicated process clear, there will be little 

 difficulty in explaining what occurs in other parts of the 

 cambium-cylinder. The cambium-cells which happen to 

 stand in the same radial row as the cells of a medullary 

 ray, simply go on being converted into cells of the medul- 

 lary ray, instead of into tracheides ; cells which differ from 

 the tracheides chiefly in retaining their living contents and 

 nutritive materials — i.e. substances like starch, proteids, 

 sugars, &c., which are used as food by the plant. Again, 

 those cells of the cambium which are divided off on the 

 outer side of the cylinder (they are always fewer in num- 

 ber) are gradually transformed into elements of the cortex, 

 and finally enter into the composition of the bark proper, 



Now and again, but much more rarely, a radial row of 

 cambial cells which, from their position, it would appear 

 should be converted into tracheides of the wood, alter their 

 destiny, so to speak, and become the originators of a new 

 medullary ray. But I must pass over these and some 

 other minor peculiarities, and refer to the illustrations for 

 further details. 



If now, instead of a log of deal, or coniferous wood, we 

 direct attention to the timber of a dicotyledonous tree, 

 such as the oak, ash, beech, chestnut, poplar, &c., 

 the differences in detail will not be found very great in 

 relation to the broad features here under consideration. 

 Turning again to Fig. I, it would be possible to select a 

 cut log of any of these timbers which presented all the 

 salient characters there exhibited. The bark would 

 present external differences in detail — such as in rough- 

 ness, colour, thickness, &c. — but it could still be described, 

 as before, as a more or less corky jacket around the whole 

 of the wood : the cut face would show the timber marked 

 by more or less numerous and prominent " annual rings," 

 traversed by smaller or larger medullary rays, radiating 

 from the central pith, and passing across the cambium 

 to the cortex. Moreover, cracks would be apt to form 

 on exposure, as before ; the opening occurring along the 

 lines of medullary rays — lines of weakness. 



Again, if we cut a segment of the wood, like Fig. 2, the 

 chief features would present themselves as there shown, 

 and the lines of demarcation indicating the annual rings 

 would be found to be due to the sharp contrast between 

 the spring wood and the autumn or summer wood, as 

 before. 



On closely examining a transverse section of such a 



piece of timber, however, we should find differences 

 which at first sight appear profound, but which on reflec- 

 tion and comparison turn out to be of more relative 

 significance, from the present point of view, than might be 

 expected. 



Selecting a given example, that of the beech for 

 instance, the first difference which strikes us (Fig. 6) is a 

 number of relatively very large openings on the transverse 

 section : these are the vessels — pitted vessels — long 

 tubular structures which are not formed by the cambium 

 of the conifers. Between these vessels are much more 

 numerous elements with very small lumina and thick 

 walls : the latter are the wood-fibres proper, and have to 

 be technically distinguished from the apparently somewhat 

 similar wood-tracheides of the pines, firs, &c. Here and 

 there, scattered in small groups, are certain rows of 

 shorter cells, which, however, are not very numerous in 

 the beech : they are called wood-parenchyma (Fig. 6, wp), 

 and occur particularly in the vicinity of the vessels. 



Fig. 6. — A piece of wood from a dicotyledonous tree (beech), supposed 

 to be magnified about loo times. Mr, a medullary ray running 

 across the transverse section : the dark band crossed by this ray is the 

 autumn wood (li), formed of clobely-crowded wood-fibres and tracheides ; 

 V, a large vessel in section : others are seen also— they are smaller and 

 fewer towards the autumn wood ; a', wood-fibres, of which most of the 

 timber is compose d ; wp, wood-parenchyma cells. 



It is beside the purpose here to describe in detail the 

 histology of the beech-wood, and reference may be made 

 to the figures for further particulars. It may suffice to 

 say that all the elements — cells, fibres, and vessels— are 

 formed as before by the gradual development of cambium, 

 cells ; and the same is true, generally, of the medullary 

 rays here that is true of those of the pines and firs, &c. 



Attention is to be directed to the fact, which is here 

 again evident, that the line of demarcation between any 

 two " annual rings " is due to the suJden apposition of non- 

 compressed elements upon closely-packed and apparently 

 compressed elements : the latter were formed in the late 

 summer, the former in the spring. Moreover, the spring 

 wood usually contains more numerous vessels, with larger 

 lumina than the autumn wood : in this particular case, 

 again, the fibres of the autumn wood are darker in 

 colour. It should be stated, however, that many dicoty- 

 ledonous trees show these peculiarities much more clearly 

 than the beech ; others, again, show them less clearly. 



