II.] WATER GIVEN OFF BY LEAVES 37 



air on the leaves, produced, for example, by a bellows, 

 increases transpiration if it does not lower the tempera- 

 ture too much, whereas a close saturated atmosphere 

 obtained by surrounding the leaves with a bottle and 

 loosely closing the mouth will reduce and almost suspend 

 the process. Syringing the leaves, again, by inducing a 

 saturated atmosphere next their surface, also greatly 

 reduces transpiration. Again, by smearing with vaseline 

 first the upper and then the under side of the leaves, we 

 shall find that with most plants the transpiration takes 

 place almost wholly from the under sides of the leaves. 

 We can demonstrate this fact in another way by taking 

 two clean but cold pieces of glass and putting a freshly 

 gathered leaf between them ; on taking them apart 

 after a minute or two, the glass will be so dewed over as to 

 form an image of the leaf, but the image will be very 

 thin and faint on the glass which was in contact with 

 the upper side of the leaf The surfaces of a leaf should 

 now be examined under a microscope ; either the leaf 

 can be looked at without any preparation by reflected 

 light, or the skin on the upper and the under sides can 

 be torn off and examined separately by transmitted 

 light. The surface of the leaf will be seen to be studded 

 with small mouth-like openings, called stomata, and 

 these openings, except with a few plants, are very much 

 more numerous on the under than on the upper side. 

 The stomata, which open and close according to the 

 illumination, the temperature, the degree of humidity of 

 the air, etc., form a means of communication between 

 the outer air and the surface of the active cells in the 

 middle of the leaf; indeed the cuticle of the leaf, except 

 for these openings, is composed of cells containing no 

 chlorophyll, and simply acts as a protecting membrane 

 practically impervious to gases. Through the stomata 

 the plant takes in the carbon dioxide it requires for 



