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 
Fia. 222,— Outline of a stoma of hellebore is abundance of water in 
in vertical section. The darker lines show the them,expand, thus open- 
shape assumed by the guard cellswhen thestoma_ .- 
is open; the lighter lines, when the stoma is ing the stoma so that the 
closed. The cavities of the guard cells with the water vapor passes out 
stoma closed are shaded, and are distinctly P p 
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. Puta 
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 
