THE GROWING PLANT 



43 



more bent or crescent shaped when turgid. Thus the 

 breathing-pore is so opened that it permits the water va- 

 por to escape by diffusion from the interior of the leaf and 

 at the same time permits the carbonic acid gas from the 

 surrounding atmosphere to enter. 



These guard-cells are thus delicately balanced valves 

 which are extremely sensitive to external influences. 

 They are open in strong light, but usually closed in 

 darkness and when the leaves are wet. They become 

 turgid as the whole leaf is turgid, . thus protecting the 

 tissues from an excess 

 of water. Conversely, 

 as the leaf loses 

 water the guard-cells 

 become less turgid and 

 close, protecting the 

 tissues from too great 

 a loss of water. In 

 this manner the plant 

 regulates the amount 

 of water in its tissues 

 according to its re- 

 quirements. The 



slightly raised spots or dots on the smooth bark of the 

 young shoots of many woody plants (lenticels) are open- 

 ings which serve a similar purpose to the stomata. 



66. The growing point. — At the tip of the stem and 

 just behind the tip of the root, is a group of cells form- 

 ing the so-called growing point. These cells divide very 

 rapidly during the growing season, and from them all 

 other kinds of tissues are developed. 



67. The vascular bxmdles. — While the plantlet re- 

 mains within the seed-case, it consists largely of cells 



y4 



Fig. 19. — Stomata (si) on leaf of Olden- 

 burg apple. Highly magnified. 



