12 THE PHYSIOLOGY OF PLANTS 



the thickening takes place we distinguish reticulate, scalari- 

 form^ spiral, or annular vessels. The fibrous elements which 

 run between the vessels and give the bundle its rigidity are 

 shown in Fig. 2 h. They are wood cells which have become 

 wedged together by their pointed ends, and which, by a chemical 

 change of their wall {lignification) have attained a great hard- 

 ness, but at the same time have become very brittle. Their 

 great length, thickness, and pointed ends distinguish them 

 from those cells which build up the soft tissues of herbaceous 

 plants, and which are usually six-sided, thin-walled, and richer 

 in contents. These latter are termed paFenehymatous cells 

 (Fig. I B, and Fig. 2 r), while the former constitute the pros- 

 enehymatous tissues. 



The hard bast cells (Fig. 2 h) are a special form of pros- 

 enchyma ; they are usually much longer than the wood cells, 

 and often much more thick walled, but they are flexible and 

 tough, and run on the outside of the woody cylinder, but 

 parallel to it. The bast used so largely in gardening opera- 

 tions consists almost entirely of such cells. These hard bast 

 cells generally form broad longitudinal bands or strips. Pro- 

 tected by these cells, and between them and the woody 

 cylinder, we find long thin-walled cells, rich in protoplasm 

 or tube-like cells, which have this peculiarity, that the trans- 

 verse walls separating one from the other are perforated by 

 pores, and they have therefore received the name of sieve-tubes. 

 These tubes are accompanied by thin-walled, bluntly ending 

 cells, the bast parenchyma (Fig. 2 hp). In like manner we 

 find between the wood vessels and wood fibres parenchy- 

 matous cells, which have, however, also become lignified ; they 

 represent the wood parenchyma (Fig. 2 hp). 



The vascular bundles which we find in plants running 

 along both the stem and the root, and branching into the 

 leaves to form a network of veins, consist therefore of two 

 very distinct portions. The most largely represented is the 

 wood, consisting of brittle lignified elements (Fig. 2, from g' to 

 hp, and h). The wood consists of vessels, narrow wood fibres 

 (lihriform), and wood parenchyma. In the case of Conifers 

 the first mentioned are replaced by wide vessel-like cells, 

 which are, however, not continuous, but resemble the wood 



