330 THE AMERICAN MONTHLY [Oct., 



lead to the spaces among the cells of the leaf and thus 

 permit gases to enter or leave. The number of stomata 

 is very great indeed, and they are very essential to the 

 operations of the organ. But the question at once arises, 

 how does the evaporation of water fail to take place 

 when the surface of the leaf is so open as this proves it 

 to he ? In figure 10 the cells of the stoma are shown to 

 contain a central protoplasmic material which it is sup- 

 posed has the function of swelling or contracting, and 

 thereby of closing or opening the entrance to the chem- 

 ical laboratory of the leaf. These guard-cells, as they 

 are called, by closing in times of atmospheric dryness 

 prevent evaporation, and in more favorable times permit 

 the equally necessary passage of gases. 



If a leaf were to be examined while it is still very 

 young, it would be composed of nearly uniform cells 

 throughout. As time elapses, the fibrovascular tissue 

 would appear developed out of some of the at first "in- 

 different" cells. Later, the chlorophyll grains or green 

 spots in the central cells would make the leaf greenish 

 in color because of their presence ; and the surface cells, 

 at first alike, would undergo differentiations, and the 

 stomata and the various hairs be formed from the ordi- 

 nary epidermal cells. 



The leaf is now seen to be made of a very large num- 

 ber of thin-walled protoplasmic cells which contain 

 chlorophyll. Their total number in the entire leaf is 

 verp great and the whole number in an entire plant 

 would almost defy numerical statement. The advantage 

 to the x>lant of such vast numbers of these cells is seen 

 when we reflect that it is only such cells as these that 

 are able to construct the substances which compose the 

 body of the plant, and that therefore these are the work- 

 ers which make the building. Thus the size attainable 

 depenbs on the numder of parenchyma cells. In a plant 

 living in the water, there would be hardly any limit to 



