THE STEM AND BRANCHES. 171 



a very rapid evaporation ; and evaporation is one 

 of their chief means of raising water from the 

 roots to the growing and living portions. 



If you examine the under side of a leaf under 

 the microscope, you will find it is covered by 

 hundreds of little pores which look exactly like 

 mouths, and which are guarded by two cells whose 

 resemblance to lips is absurdly obvious. These 

 pores are commonly known to botanists by the 

 awkward name of stomata, which is the Greek for 

 mouths; and mouths they really are to all exter- 

 nal appearance. You must not suppose, however, 

 that they are truly mouths in the sense of being 

 the organs with which the plant eats ; the upper 

 surface of the leaf, as we saw, with its layer of 

 water-cells and its assimilating chlorophyll-bodies, 

 really answers in the plant to our mouths and 

 stomachs. The stomata or pores are much more 

 like the openings in the skin by which we per- 

 spire ; only perspiration or evaporation is an even 

 more important part of life to the plant than it is 

 to the animal. Each of the stomata opens into an 

 air-cavity; and through it the liquid evaporated 

 from the cells passes out as vapour into the open 

 air. Many leaves have thousands of such pores 

 on their lower surface ; they may easily be rec- 

 ognised under the microscope by means of the 

 curious guard-cells which look like lips, and which 

 give the pores, in fact, their strange mouth-like 

 aspect. 



What is the use of these lips ? Well, they are 

 employed for opening and closing the evaporat- 

 ing pores, or stomata. In dry weather it is not 

 desirable that the pores should be open, for then 

 evaporation should be limited as far as possible. 

 So, under these conditions, the lips contract, and 



