MORPHOLOGY 



131 



living elements in the wood strands always bears a distinct relation to 

 the water-courses which they accompany, enclosing them in a more or 

 less complete sheath. The living cells adjoining the tracheal elements 

 are in communication with them by means of one-sided bordered pits. 

 When such pits occur in living cells the pit cavities are absent, but 

 present in the case of tracheal elements ; they differ from the true 

 bordered pits in the absence of a torus on the pit-closing membrane, and 

 in being unlignified. No communication exists between the tracheal 

 elements and the dead wood fibres ; in cases where they adjoin each 



mk;^^A 



Fig. 146.— Portion of a transverse section of the wood of Tilia parvifolia. m, Large pitted vessel ; 

 t, tracheids ; I, wood fibre ; p, wood parenchyma ; r, medullary ray. ( x 640.) 



other there are either no pits developed or they are extremely small 

 and few in number. 



The elements of the bast strands of Gymnosperms and Dicoty- 

 ledonous woody plants may be referred, just as in the case of the 

 wood strands, to two distinct forms of tissue, the sieve-tube and the 

 parenchymatous portion. The former is composed of sieve-tubes, or 

 sieve-tubes with their companion cells. Its function is the conduction 

 of proteid material ; that of the parenchymatous tissue, on the other 

 hand, is the conduction of the carbohydrates and the absorption of the 

 by-products of metabolism. The phloem tissues remain functional only 

 a short time, they afterwards lose their contents and become, for the 

 most part, crushed and disorganised. 



