86 



Introduction to Botany. 



that, while the leaf is covered with an epidermis which 

 keeps the water from passing out of it too rapidly, there 

 are openings in the epidermis (called stomata ; singular, 

 stoma) through which gases may pass in and out. (See 



Fig. 35-) 



Observations 90 and 93 have taught us that carbon diox- 

 ide is necessary to the manufacture of starch by the leaf, 



and that if the stomata are 

 artificially closed, starch can- 

 not be produced, although all 

 other conditions may be fa- 

 vorable. From this we may 

 conclude that the stomata are 

 the ways through which the 

 carbon dioxide of the atmos- 

 phere enters the leaf. Thus 

 we see the leaf is well provided 

 for receiving the raw materials 

 upon which it must work. But 

 how is the energy of the sun- 

 light employed in transform- 

 ing the raw materials into the 

 finished food product ? 



If we strip off a bit of the 



epidermis, without bringing with it the underlying tis- 

 sues, and hold it between us and the light, we see that a 

 large percentage of the light passes through it ; to such 

 an extent, in fact, that objects may be seen through it. 

 This fact assures us that the light may pass freely into 

 the interior of the leaf. If now we hold an entire leaf 

 between us and the light, we find that by far the larger 

 part of the light has been absorbed by the leaf ; in 

 other words, most of the energy from the sun which 



FIG. 35. 



Epidermal cells with heavy outlines. 

 The stomata are the elliptical bodies 

 consisting of two curved guard cells 

 with a narrow opening between. 

 Ends of palisade cells of circular 

 outline are seen beneath the epider- 

 mis. From leaf of Solanum rostra- 

 turn seen from above. Drawn with 

 a camera lucida. 



