266 TIMBEES AND THEIR USES 



excellent textbooks of botany which, are published 

 in this country. 



We have said that the function of the central 

 portion of the ordinary plant stem is two-fold. 

 Its mechanical function is to support the stem. 

 Its physiological function is to form a channel 

 for the transport of raw food material from root 

 to leaf, and of elaborated food material from 

 leaf to growing points, storage organs, etc. We 

 should expect to find, therefore, that the tissues 

 of this central portion, or stele,, are strong and 

 that they are so constructed that hquids may 

 pass along them. Examination will show that 

 our surmise is correct. 



The greater part of the tissues of the stele 

 is composed of vessels, and a vessel botanically 

 is an elongated cell whose end walls have broken 

 down entirely or in part so that, with its neigh- 

 bours at either extremity, the cell forms a long 

 tube. Not aU the cells of the stele are vessels, 

 many of them retain their transverse waUs. There 

 are, again, in the stele a number of elongated 

 cells whose cell walls have become hard and 

 thickened ; it is these cells, called fibres, which 

 lend strength to the stem. In the wood tissue 

 we find that cells, fibres and vessels all have 

 reinforced walls, brought about by the cellulose 

 of which the young cell walls were composed 

 becoming impregnated with a substance called 



