100 



BOTANY: PRINCIPLES AND PROBLEMS 



occur in the phloem, and some parenchyma is usually present 

 there also. 



Wood. — The inner portion of the fibro-vascular cylinder 

 consists of the wood or xyleyn, which provides mechanical rigidity 

 for the stem and transports the stream of water and dissolved 

 substances from root to leaf. As essential elements in the xylem 



Fig. 52. — Types of cells found in wood. A, fiber. B, tracheid. C, vessel- 

 cell, with ladder-like end wall. D, vessel-cell, with completely open or porous end 

 wall. E, very short, broad, vessel-cell. F, two ray-cells. G, two vertical wood- 

 parenchyma cells. 



we find cells which are much elongated parallel to the main axis 

 of the stem and in which the cellulose walls have become very 

 thick and woody (Fig. 52, A, B, C, D and E). Such walls are 

 said to be lignified. As soon as one of these cells is fully devel- 

 oped, it dies and its protoplasmic contents disappears, so that only 



