] 70 PART III. PHYSIOLOGY. [41 



of form is that different plants attain the most favourable position 

 of their foliage-leaves and reproductive organs in different ways 

 which depend upon the particular combination of external condi- 

 tions under which they severally have existed. 



The internal structure of the stem varies to some extent with its 

 general habit, and mainly in the arrangement and relative degree 

 of development of the sclerenchyma ; thus, the sclerenchyma is more 

 largely developed in an erect than in a trailing perennial stem. 



There is one point in connexion with the relation of the vascular 

 tissue of the stem to the leaves which require special considera- 

 tion. It has been pointed out (p. 149) that vascular tissue is formed 

 secondarily in the stems (and roots) of most Dicotyledons and 

 Gymnosperms, whereas it is not so formed in those of most Mono- 

 cotyledons and Vascular Cryptogams. A consideration of the 

 general habit of the plants in question at once affords a clue to 

 this remarkable diversity. In the plants of the former groups, 

 the stem, as a rule, branches considerably, and consequently there 

 is every year an increase in the area of the leaf-surface of the 

 plant ; whereas in the plants of the latter groups, the stem 

 branches but little if at all, and the area of leaf-surface remains 

 approximately constant in the adult plant. It is clear that, in the 

 former case, the increase of leaf-surface necessitates an increase 

 in the conducting vascular tissue, a demand which is met by 

 the annual formation of an ever-widening ring of vascular tissue 

 by the cambium. Hence, in a plant of this kind, the vascular 

 bundles in the leaves of any one year are continuous, in the stem, 

 with the new vascular tissue formed in that year by the cambium. 



Stems may be specially modified both in external form and 

 internal structure for the performance of special functions. Thus, 

 in leafless plants, in which the stem or its branches have to dis- 

 charge the functions of the leaf, they may become phylloid ; that 

 is, it may assume a flattened, leaf-like appearance (p. 28). The 

 cortical ground-tissue of the stems of such plants resembles the 

 mesophyll of foliage-leaves, not only in that the cells contain 

 chlorophyll-corpuscles in abundance, but also in the more or less 

 complete differentiation of a superficial palisade-layer from a more 

 deeply placed spongy tissue. 



Again, stems may be specially modified to serve as depositories 

 of reserve materials (e.g. tubers of potato), or of water (e.g. stems 

 of Cactaceae), when they are much thickened by the development 

 of a large quantity of parenchymatous ground-tissue, in the cells 



