166 



PRACTICAL COURSE IN BOTANY 



FIG. 222. Outline of a stoma of hellebore 

 in vertical section. The darker lines show the 

 shape assumed by the guard cells when the stoma 

 is open ; the lighter lines, when the stoma is 

 closed. The cavities of the guard cells with the 

 stoma closed are shaded, and are distinctly 

 smaller than when the stoma is open. 



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 

 is abundance of water in 

 them, expand, thus open- 

 ing the stoma so that the 

 water vapor passes out 

 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 wither, the guard 

 cells, losing their water, collapse, closing the pore, and trans- 

 piration 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' m , 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 



