The Inner Structure of the Plantlet. 51 



thus forming an opening through which water escapes 

 and carbonic acid enters the leaf. 



These guard-cells are 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, protect- 

 ing 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 requirements. 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. 



66. The Growing Point. At the tip of the stem 

 and just behind the tip of the root, is a group of cells 

 forming the so-called growing point. These cells di- 

 vide very rapidly during the growing season, and from 

 them all other kinds of cells are evolved. 



67. The Vascular (vas'-cu-lar) Bundles.* While the 

 plantlet remains within the seed-case, it consists largely 

 of cells more or less cubical or globular in outline. But 

 germination scarcely commences before some of the 

 cells begin to increase greatly in length without a cor- 

 responding increase in thickness. f These elongated 

 cells form in groups or bundles (vascular bundles) 

 that extend lengthwise through the stem and roots, and 



* Also called flbro-vascular bundles. 



t Cells of the former class, i. e., those that retain their globu- 

 lar shape, are called Parenchyma (pa-ren'-chy-ma), and those of 

 the latter class prosenchyma (pro-sen'-chy-ma) (Fig. 20). Fig. 17 

 shows parenchyma cells from the appple leaf. 



