50 



Principles of Plant Culture. 



The guard-cells are delicately-balanced valves which 

 are extremely sensitive to external influences. They are 



open in strong 

 light, but usu- 

 ally closed in 

 darkness and 

 w hen the 

 leaves are wet. 

 The water es- 

 caping from 

 leaves (75), 

 and the car- 

 bonic acid en- 

 tering them 

 (62) mostly 



FIG. 18. Showing stomata (st.) on leaf of the garden paSS through 

 beet. Moderately magnified. (After Fank and Tschirch). 

 See also Figs. 15, 19 and 22. the Stomata. 



The slightly-raised spots or dots on the smooth bark of 

 the young shoots of many woody plants, (lenticels (len'-ti- 

 cels)), serve a similar purpose to the stomata. 



67. The Growing 

 Point. At the tip of 

 the stem and just be- 

 hind the tip of the 

 root, is a group of 

 cells forming the so- 

 called growing point. 

 These cells divide 

 very rapidly during 

 the growing season, 

 and from them all 



FIG. 19. Showing stomato (st.) on leaf of 

 Oldenburgh apple. Highly magnified. 



other kinds of cells are evolved. 



