678 PART IV. THE PHYSIOLOGY OF PLANTS. 



followed 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 in- 

 capable of any vital action inasmuch as it contains no protoplasm. 



A fundamental anatomical fact connected with the conduction 

 of liquids by the wood is that the functional tracheal tissue is 

 destitute of intercellular spaces, and is, in fact, shut off from all 

 communication with the external air. This occlusion is ensured, 

 generally speaking, by the endodermis (p. 165); but in stems 

 and roots which have grown in thickness, and in which the 

 endodermis has been disorganised or thrown off, the compact 

 bast-tissue replaces it functionally. 



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. 135). 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 

 experiment : If a ring of tissue, extending inwards as far as the 

 cambium, be removed from the trunk of a young dicotyledonous 

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

 interrupted. The effect of this upon the tree is that the portion 

 of the trunk below the wound, and the roots, cease to grow and 

 slowly die, whereas the trunk and branches above the wound 

 remain healthy and continue to grow until the roots are no longer 

 able to absorb water, etc., from the soil with sufficient activity. 

 Inasmuch as the cortical tissue, through which the sugar travels, 

 is necessarily also cut through, the operation deprives the lowei 

 parts of the body of the whole of their supply of organic plastic 

 material from the leaves, but does not interfere with the conduc- 

 tion of water from the roots to the leaves. 



The sieve-tubes differ from the vessels of the xylem in that 

 they contain living protoplasm ; their function is therefore 



