NATURE OF PLANTS 61 



number of cells in the root exerts a pressure that often amounts 

 to considerably more than one atmosphere, i. e., 15 Ibs. to the 

 square inch. It is the steady expressing of fluids by these cells 

 that causes the familiar phenomena of the "bleeding," or flow 

 of water from stumps in the spring; likewise the "bleeding," of 

 injured branches, or the flow of sap, for in the stem the living 



FIG. 36. Cross-section of root taken above section shown in Fig. 33: e, 

 epidermis with root hairs; c, cortex bounded on inner side by endodermis. 

 end. Within is the central region containing vascular bundles; *, xylem; p. 

 phloem. I. D. Cardiff. 



cells are constantly absorbing and giving off water as in the root. 

 In the summer, stumps and stems do not "bleed" as a rule be- 

 cause the water lost by transpiration nearly empties the cells, 

 whereas in the spring before the leaves appear they become filled 

 with fluid. 



26. The Sensitiveness of the Root. We may now ask how 

 does this elaborate root mechanism become so perfectly adjusted 

 to the soil. If the root of a pea or bean seedling is placed hori- 

 zontally in sawdust, after one or two hours it will begin to 

 curve down toward the earth center. No matter in what position 

 it is placed the result is always the same. We are so familiar 

 with the downward growth of roots into the soil that we never 



