166 PRACTICAL COURSE IN BOTANY 



Figs. 218, 219, that when the ring is strongly inflated, it 

 will expand, and in enlarging its own circumference, will at 

 the same time increase the diameter of the opening in the 



center. When the ex- 

 pansive force is removed, 

 it collapses, thus closing, 

 or greatly reducing, the 

 aperture. 



In the same way the 

 guard cells, when there 



Fig. 222.-Outline of a stoma of hellebore '^ abundance of Water in 



in vertical section. The darker lines show the them, expand, thuS OpCU- 



shape assumed by the guard cells when the stoma • fV, f fVi f fV> 



is open ; the lighter lines, when the stoma is l^^S ^'^^ StOma SO tnat me 



closed. The cavities of the guard ceUs with the water vapor pasSCS OUt 

 stoma closed are shaded, and are distinctly 



smaller than when the stoma is open. more readily. But when 



there is a dearth of 

 moisture, or when, by reason of chemical action in the soil, 

 the roots fail to supply it, the leaves wilt, the guard cells, 

 losing their water, collapse, closing the pore, and transpira- 

 tion is thus prevented or greatly retarded. (Fig. 222.) 



Sketch a portion of the epidermis as it appears under the mi- 

 croscope, labeling the parts. If stomata can be found in both 

 conditions, make sketches showing them both open and closed. 



184. Internal structure of a leaf. — Roll a leaf blade, or 

 fold it tightly to facilitate cutting, and with a scalpel, or a very 

 sharp razor, cut the thinnest possible slice through the roll. 

 This will give a section at right angles to the epidermis. 

 It should be so thin as to appear almost transparent. Put a 

 small bit of a section in a drop of water on a slide, place under 

 the microscope, using a high power, and look for the parts 

 shown in Fig. 223. Notice the horizontally flattened cells of the 

 upper epidermis, e, and of the lower epidermis, e' ; also the ver- 

 tically elongated palisade cells, p, filled with particles of green 

 coloring matter. These particles are the chlorophyll bodies, 

 to which the green color of the leaf is due. They are the 

 active agents in the manufacture of plant food, and in a leaf 



