io8 



SCIENCE-GOSSIP. 



moreover, get sometimes very thick, and by a 

 corresponding diminution in the number of the 

 vessels ; while some of the medullary rays are much 

 larger, showing straighter and more elongated 

 cells ; the density of the autumn and that of the 

 spring wood is considerably different. 



Let us now endeavour to interpret the real 

 meaning of these important structural or histo- 

 logical changes and variations. The question as 

 to whether the tree has sought and found the 

 habitat, or the habitat has modified the tree, 

 cannot be discussed here. We must, in the first 

 place, signalize the precise physiological function 

 of each of the woody elements which we have 

 specified above. These elements are (i) the vessels 

 which serve as conducting pipes for the water and 

 the matters which it holds in solution from the 

 roots of the tree up to the leaves. In fact, from 

 the ultimate ramifications of the roots even up to 

 the finest ramifications of the vascular bundles in 

 the leaves, the vessels of the living outer wood 

 form a continuous system, whose proper function is 

 to conduct water. In them the water filters from 

 one to another, not in the direction of their length, 

 but through tiny pits with a thin layer of cellulose 

 at the bottom which they each bear on their side- 

 walls, leaving small globular spaces between the 

 adjacent elements for intercommunication. In 

 this way, through minute funnels and filters, the 

 water moves in the cavities (not in the walls) of 

 the vessels and across the shut enclosures which 

 separate them. It is impelled to do so ever 

 upwards, and scarcely, if at all, sideways, by virtue 

 of forces partly vital and partly physical. (2) The 

 tracheides perform the same function, and prac- 

 tically in the same way, by means of bordered pits; 

 but, of course, these elements being smaller and 

 narrower, their efficacy is not so conspicuous. 

 (3) The libriform fibres sustain a supporting role, 

 for which purpose their walls are very thickened 

 and lignified ; sometimes, indeed, they are almost 

 quite solid, and have only extremely small simple 

 pits, frequently none at all, so that as a rule 

 they cannot serve as water conduits. (4) The 

 wood parenchyma (including the cells of the 

 medullary rays) only serve as reservoirs for food 

 material, such as protoplasm, starch, sugar, oil, 

 etc., which is used up in the course of time for the 

 nourishment of the tree. The cells, especially of 

 the outer rings of the wood, are generally full of 

 this commissariat provender ; so that, although they 

 dispense some of this to the water current ascend- 

 ing in the vessels near which they always lie, yet 

 they do not themselves take an active part in the 

 actual transmission of the aqueous element. In 

 fact, their greater or lesser development is in no 

 positive or direct way dependent on the mode of 

 life of the tree. 



Now, bearing in mind the several functions of 



these respective elements of the woody tissue^ 

 and considering their variations as regards form 

 or size corresponding with their surroundings, 

 perhaps we can understand the mutual relation- 

 ship of the former feature to the latter. Thus, 

 for instance, where a tree grows in a place where 

 damp air and moist soil prevail there on account 

 of the liberal supply of water which is bestowed on 

 it, it requires in its stem and branches large, 

 numerous and roomy water-conducting apparatus. 

 This need is most efficiently supplied by the in- 

 crease of the number and size of the vessels and 

 tracheides, as in willows, poplars, alders, etc. 

 Moreover, the volume and permanency of the water 

 supply, by enabling the plant to always have a 

 sufficient supply of food, render unnecessary 

 the existence of any special storehouse of 

 reserve therefor, such as the wood parenchyma 

 provides. Hence this element of its tissue is 

 comparatively feebly developed, it being small and 

 narrow in all its dimensions. For similar reasons 

 the wood fibres are few, partly because of the great 

 predominance of the other elements, or partly 

 because extensive thickening and lignification of 

 the cell-walls are not possible or requisite in a 

 denizen of a damp habitat. On the other hand, 

 when we examine the tissues of a tree that prospers 

 in a dry station, we detect in the relatively small 

 number of its vessels and tracheides a much lesser 

 need for the transmission of water. The fibres are 

 more numerous and robust because there is more 

 room for them, and the reserve elements are more 

 extensive, possibly to meet the greater precarious- 

 ness of the food supply. Nay, these modifications 

 may be even carried further still : an elm, for 

 instance, growing in a dry, arid, calcareous soil, 

 will exhibit very thick fibres and ample medullary 

 rays. In this case the difference between the first 

 and the last formed wood in the annual ring will be 

 very decided. Trees which are introduced to well- 

 cultivated land will betray likewise the characteristic 

 histological features which attend a transference 

 from damp and swampy localities to those where 

 the water supply is relatively scanty. 



Thus then we have come to understand how, 

 in harmony with the intensity, so to speak, of the 

 forces operating from without (the conditions 

 of the environment), the plastic cells of the embry- 

 onic tissues are correspondingly moulded, though 

 apparently in the same direction, yet subject, 

 nevertheless, all the time to the specific needs and 

 exigencies of the organism. Indeed, it has been 

 held that : " in the same plant the same young 

 cell may be modified in its normal development 

 by external conditions so as to yield by self- 

 differentiation either a lignified fibre (a supporting 

 element), or a soft cell with nutritive contents 

 (a reserve element)." 

 Pattcrdalc, Westmorland. 



