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 

 when the 

 leaves are wet. 

 The water es- 

 caping from 

 leaves (75), 

 and the car- 

 bonic acid en- 

 tering them 

 (62) mostly 



Fig. 18. Showing stoma ta(.s-/.) on leaf of the garden pgjgg through 

 beet. Moderatelymagnifled. {After FankandTschlrch). 

 See also Figs. 1.5, 19 and 22. tllC 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 

 other kinds of cells are evolved. 



W 



Fig. 19. Showing stomato [st.) on leaf of 

 Oldenburgh apple. Highly magnified. 



