THE WORK OF STEMS 229 



water under a cover -glass and examine with the high 

 power. We see that both the ducts (d, Fig. 133) and 

 some of the smaller cells of the wood are covered with 

 regular markings, round or elliptical in shape ; these 

 are thin places in the walls called pits (p. Fig. 138) 

 through which water may pass readily from one cell 

 to another. The smaller cells of the wood are of two 

 kinds ; short cells with blunt ends (called wood paren- 

 chyma,^), and longer cells with pointed ends (called 

 tracheids, tr) ; these latter have thin walls with spiral 

 or annular (i.e., ring-shaped) thickenings. The spiral 

 and annular thickenings of the cell- wall are exceedingly 

 useful to prevent the cells from being crushed by the 

 pressure of the surrounding cells and so rendered use- 

 less for the conveyance of water. As we follow the 

 bundles up into the tip of the stem, we notice that 

 the spiral and annular tracheids are the first elements 

 of the wood to be formed, and that their structure 

 allows them to stretch so as to keep pace with the 

 growth in the length of the stem, since they are 

 formed in the growing region of the stem near the tip. 

 The thick-walled elements of the wood can stretch 

 little or not at all, and they acquire their thick walls 

 (and characteristic structure) in the region just below 

 the growing portion of the stem where elongation has 

 practically ceased (see page 249) . 



Can you explain why the water rises so much more 

 rapidly in the ducts than in the other cells ? Not only 



