THE DESCENDING SAP 



125 



as " lips " is not fanciful, for 

 the orifice between them is a 

 veritable mouth, and the name 

 which physiologists apply to 

 such openings is stomata, which 

 is simply the Greek word for 

 " mouths " (figs. 160, 162, 163). 



The stomata, indeed, are 

 little mouths or crevices in 

 the epidermis, caused by the 

 separation of certain cells in 

 the course of growth; and 

 these cells form the "lips" of 

 the mouth, and are known as 

 guard-cells. Each is shaped 

 like a crescent, their points or 

 horns meeting to form the 

 stoma or mouth ; and it is by 

 means of these peculiar struc- 

 tures that the plant tran- 

 spires. The tiny openings 

 establish a communication 

 between the atmosphere and 

 the air chambers or inter- 

 cellular spaces in the interior 

 of the plant, the passing in 

 and out of gases being regu- 

 lated in a beautiful manner 

 by the guard-cells. 



Fig. 164 represents a minute piece of the epidermis of the Madder-plant 

 (Rubia tinctoria^ in which three of the stomata are plainly shown ; but far 

 better for examination is a beautiful plant of the Daffodil order Amarylli- 

 daceaa known by the very ugly name Hippeastram. It is a variety of 

 H. ackermanni, one of the largest flowering species' of the genus. As a 

 rule the clefts vary in length from -j^^^h. to rWircn^h f an inch, and so 

 abundant are they on some leaves that a square inch of tissue may contain 

 as many as 250,000 of them. They are met with only in those parts of the 

 plant where they are actually needed, for our protoplasts work on economic 

 principles, never wasting their forces. Hence you will look in vain for the 

 stomata on leaves which grow under water,* or on the under surface of 



* Water stomata, however, are found in some plants. " These are situated over the ends 

 of small masses of specially modified parenchymatous cells (glandular cells), in which vascular 

 bundles terminate, and which are known as water glands." Water stomata give off water and 

 various substances in solution. Text-book of Biology, p. 92. 



Photo by-] \E. Step. 



FIG. 159. RHODODENDRON (R. arboreum). 



This magnificent tree, a native of the Himalaya, is quite hardy in 



some parts of southern England, and reaches great proportions. The 



clump photographed is about twenty feet in height. 



