CHAPTER I. GENERAL PHYSIOLOGY. 677 



is most perfectly developed in the stems of land-plants which 

 grow erect and have to support the weight of many leaves and 

 branches : whereas in water-plants the development of stereom 

 is rudimentary, for their stems, being supported by the water, do 

 not need to be highly rigid. 



When it is developed in the walls of fruits or in the seed- 

 coats, the sclerenchymatous tissue serves to protect the seed from 

 being eaten or digested by animals. 



d. The Tracheal Tissue of the Xylem (see p. 133). It is clear 

 that when a plant-body is massive, partly subterranean and partly 

 subaerial, there must be some means for readily distributing the 

 water and other substances absorbed by the root. This dis- 

 tribution may take place by diffusion from cell to cell ; and, as a 

 matter of fact, this mode of distribution suffices in some plants in 

 which the seat of absorption is not far from that of consumption 

 (e.g. larger Fungi and Algae). But when these points are widely 

 separated, special conducting-tissue, in the form of the tracheal 

 tissue of the xylem, is differentiated. 



The function of this tracheal tissue is demonstrated by the 

 following experiment. If a cut be made all round the stem of a 

 dicotyledonous tree, to such a depth as to penetrate far into the 

 xylem, the effect is that the leaves, borne on the stem and its 

 branches above the incision, will soon droop and wither. This is 

 due to loss of water, in consequence of which the cells of the 

 leaves lose their turgidity, and the leaf-blades and petioles are no 

 longer sufficiently rigid to maintain their position of expansion. 

 The loss of water is the result of the continuance of transpiration 

 in the absence of a supply of water to meet it. The incision 

 which has destroyed the continuity of the wood has also cut off 

 the supply of water from the root. The relation between the 

 development of the xylem and the activity of transpiration is well 

 illustrated by the comparison of the vascular bundles of a land- 

 plant with those of an allied submerged aquatic species. The 

 former transpires actively and has well-developed xylem : the 

 latter does not transpire at all, and has quite rudimentary xylem. 



Conduction takes place in dicotyledonous tree-trunks only 

 through so much of the peripheral portion of the wood as includes 

 living parenchymatous cells. The thickness of this conducting 

 region varies widely ; it is relatively small where the wood is 

 sharply differentiated into alburnum and duramen (see p. 199), 

 and in such trees (e.g. Oak) section of the alburnum is soon 



