THE FLOWERING PLANTS. 21 



in which the stipules have developed into spines. Find 

 cases in which the stipules are united and perhaps encase 

 the petiole to some extent. 



18. Tear off small bits of the epidermis from the upper 

 and under sides of a leaf, the Lilac leaf, for instance. 

 l j lace it on a microscope slide in a drop of water, outside 

 surface uppermost, and place a cover glass over it. Ex- 

 amine with the low power of the compound microscope 

 for stomata, or minute openings into the interior of the 

 leaf. Each stoma is inclosed by 

 special cells, the guard cells. The 

 stomata are very important organs. 

 The little cells between which the 

 openings pass open and close under 

 the changes of moisture contained in 

 the plant and under the influence of 

 light. They control the transpiration 

 of the water from the plants. Trans- 

 piration is the name given to the 

 passing of the water out of the plant 

 into the air ; it is more than simple FIG. 2. stomata of Lilac 

 evaporation, for its amount depends 



upon the pressure of the fluids within. Root pressure 

 and the control of the transpiration by the stomata regu- 

 late the supply of food materials brought up from the 

 soil by the sap, according to the necessities of the plant. 



On which side of the leaf do you find stomata more 

 abundant ? Make similar studies of the stomata of other 

 leaves. Draw portions of several specimens, showing the 

 stomata and the superficial cells of the epidermis. Are 

 the stomata invariably more abundant on either the upper 

 or under side of the leaf ? If so, on which side ? 



Examine some of the stomata under high power. Draw, 

 showing stomata and epidermal cells. 



Through the epidermis of the leaf, mainly that on the 



