684 PART IV. THE PHYSIOLOGY OF PLANTS. 



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 -surf ace 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 the stem or its branches may become phylloid ; 

 that is, it may assume a flattened, leaf-like appearance (p. 44). 

 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. The reduction of the foliage- 

 leaves and the compensating development of the stem is a charac- 

 teristic of many xerophilous plants : under these circumstances 

 the stem generally becomes succulent (as in Cactaceee, p. 628, and 

 some Euphorbiacese) owing to the development of a large amount 

 of parenchymatous tissue. 



Again, stems may be specially modified to serve as depositories 

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

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

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

 of which the water or the reserve-materials are deposited. Or 

 they may be developed into thorns (p. 44) as a protection against 

 being eaten. 



The special physiological properties of stems by which they 

 assist in placing the foliage-leaves and reproductive organs in the 

 most favourable position, are their various kinds of irritability to 

 the action of external stimuli. 



