52 GROUPS OF TISSUES, OR TISSUE SYSTEMS 



each other at both ends. When these guard cells become 

 more turgid they curve outward, thus opening the stoma, 

 while a loss of turgidity results in its closure. Usually 

 the stomata open while the plant is in the light and close 

 partly, sometimes completel}^, in darkness. An excessive 

 loss of water by the plant reduces the turgor of the guard 

 cells, overcoming the effect light has in opening the 

 stomata, and causes them to close, thus conserving the 

 moisture in the plant. 



76. Stomata occur on aerial leaves and stems and 

 more rarely on flowers and fruits. On underground stems 

 and leaves they are less abundant (and often not func- 

 tional), Avhile they are wanting on roots. On submerged 

 parts of aquatic higher plants they are lacking or only 

 rudimentary. On leaves they are usually more abundant 

 on the lower than on the upper surface. The numbers 

 as well as size of the stomata vary greatly for different 

 species. The following table will give an idea of their 

 relative abundance in some plants. (Page 53.) 



Laboratory Studies, (a) Strip off the epidermis from the 

 upper and lower surfaces of the leaves of various plants, and 

 mount with the outer surface upward. If air bubbles 

 interfere, add alcohol, and follow this by a weak potash 

 solution, to swell the tissues again. Leaves of various grasses 

 or of carnation will show epidermal cells much elongated, while 

 more isodiametric cells may be found on the leaves of such 

 plants as the live-for-ever (Sedum or Sempervivum), dock 

 (Rumex), cabbage, etc. 



(b) In the same specimens that were used for the foregoing, 

 study the stomata and their relations to the adjacent cells. 

 Compare the numbers of stomata on the two sides of the leaf, 

 and their relative size and number on different species of plants. 



(c) Cut cross-sections of various leaves. Those of cabbage 

 and carnation, as well as of many other plants that grow in dry 

 regions, will show a considerable development of cuticle. Note 

 the structure of the stomata as shown in cross-section, and their 



