64 TISSUES 



the vegetable organism, just as in human society; moreover, even the 

 most intense specialisation in relation to one particular function 

 scarcely ever renders a tissue utterly unfit for any other form of 

 physiological activity. It is therefore hardly surprising to find that a 

 tissue may be capable of undertaking one or more subsidiary functions. 

 Thus the fact that both collenchymatous cells and bast-fibres are 

 primarily mechanical elements does not prevent them from developing 

 photosynthetic chlorophyll-corpuscles (collenchyma) or from serving as 

 repositories of starch (bast-fibres). 



The anatomical characters connected with subsidiary functions often 

 in no wise interfere with those which pertain to the principal function. 

 In the case of bast-fibres, for example, the elongated spindle-shaped 

 form, and the thickened condition of the walls, are quite unaffected by 

 the deposition of starch in the cavity. In other cases, indeed, the 

 leading anatomical features may undergo a certain amount of modifica- 

 tion in the interests of a subsidiary function, although the alteration 

 produced in this way must always be restricted within comparatively 

 narrow limits. An example will serve to elucidate this point. The 

 specialised photosynthetic elements of terrestrial green plants are more 

 or less cylindrical in shape, and are usually located immediately 

 beneath the upper epidermis of the leaf, where they constitute the 

 so-called palisade-tissue. The walls of these cells are soft and 

 unthickened throughout, a feature which is the anatomical expression 

 of the very active interchange of material that is continually going on 

 between the palisade-cells and the adjacent tissues. Now there are 

 certain Ferns, such as Didymochlacna sinuosa, in which the leaf is 

 devoid of an upper epidermis in the physiological sense. Here the 

 palisade-tissue, which in other plants is sub-epidermal, has become 

 superficial, and in fact forms a part of the outer covering of the leaf. 

 In these circumstances, it must evidently act as a dermal tissue, 

 besides performing its primary photosynthetic function ; as a matter of 

 fact, the outer walls of the palisade-cells are in such cases always 

 slightly thickened, and also cuticularised. Here a leading histological 

 feature of the palisade-cell, namely, the unthickened condition of 

 its cell-wall, is modified in the interests of the secondarily accp:iired 

 subsidiary function of protection ; but the change of structure in no 

 way interferes with the principal function of the palisade-tissue. 



The preceding remarks will have made it sufficiently clear that the 

 anatomical and topographical principles underlying the construction of 

 all tissues and tissue-systems are determined by the functions which 

 the tissues are destined to perform. In discussing these principles 

 more fully, it would obviously lie inadvisable to consider the topo- 

 graphy of the tissues separately from their anatomy and histology ; 



