40] CHAPTER I. GENERAL, PHYSIOLOGY. 165 



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 young 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 com- 

 parison 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. 143), and 

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

 lowed by the withering of the leaves above the wound ; it is more 

 considerable in trees like the Beech, in which the transition from 

 alburnum to duramen is gradual ; and it is most extensive in those, 

 such as Birch and Maple, in which there is no differentiation of 

 alburnum and duramen. The dead portion of the wood does not 

 conduct, but at most only serves as a reservoir of water. 



The tracheal tissue of the xylem discharges a purely mechanical 

 function in connexion with the conduction of water ; it is incapable 

 of any vital action inasmuch as it contains no protoplasm. 



The liquid conducted from the roots to the leaves by the tracheal 

 tissue is not pure water, but holds in solution substances absorbed 

 by the roots from the soil ; hence this tissue plays an important 

 part in the distribution of food-materials in the plant. 



e. The Sieve-Tissue (see p. 94). The function of the sieve-tubes 

 or phloem-vessels is to convey proteids from the organs in which 

 these substances are deposited or are being formed, to other parts 

 in which they are either being consumed or deposited as reserve 

 plastic material. This is demonstrated by the following experi- 

 ment : If a ring of tissue, extending inwards as far as the cam- 

 bium, be removed from the trunk of a young dicotyledonous tree, 

 the sieve-tubes will all be cut through, and their continuity inter- 



