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 completely, 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), while 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, ete. 
(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 
