THE BREATHING PORES OF LEAVES. 131 
lution on a section like fig..9 the thick outer wall of the epider- 
mis, especially in the outer half, turns deep brown, a color whic 
follows down the sides of the cavity, extends as a very thin layer 
through the slit, and fades out on the walls of the rectangle below ; 
the substance thus colored is cork, or of the nature of cork. The 
main portion of the walls of the pore cells, and all the cellular 
tissue underneath, become violet or purple; the reaction of cel- 
lulose. The little grains in the cavities of the pore cells are of a 
bright blue, betokening starch; and the granular mass of proto- 
plasm in which these are embedded becomes yellowish brown. 
In studying the development of this complex organ, we take the 
youngest leaf of the plant, and find on its base (the youngest 
portion) no trace or hint of stomata. A very little higher up we 
find the epidermis appearing as in fig. 1, many of the cells hav- 
ing built a partition across their front end cutting off about a 
quarter of the original cells. .These small cells are distinguished 
from the remaining portion of the originally single cells, and from 
the undivided cells, by being filled full of granular protoplasm 
while the other cells are only partially filled with the protoplasm 
constituting the nucleus. These little cells, called mother cells, 
soon grow so as to become longer than broad, and are raised by 
the more rapid growth of the surrounding cells so as to leave an 
air space below (figs. 2 and 4). An approach to a spherical form 
is now made by the mother cells, and the walls of the neighboring 
cells are a little thickened with the deposition of cork substance 
giving the first trace of the thick-walled square of the ripe sto- 
mata. Next the mother cell divides by the formation of a thin 
` partition which runs in the direction of the point of the leaf, and 
is perpendicular to its surface. Soon this partition thickens in 
the middle (fig. 3) and splits through the thickened portions to 
within about a fifth of each end. All further growth only effects 
minor changes in the form of the cells, or an increased thickening 
of their wall. Figs. 4, 5, and 9 show the various stages of growth 
in cross section, and fig. 10 in longitudinal section. 
In Tradescantia discolor the stomata, quite different in ap- 
pearance, are more readily seen from the surface. (Fig. 14.) 
The peculiarity of these stomata consists mainly in the structure 
and form of the epidermis cells immediately around them and 
constituting a part of the stomata apparatus. The form and ar- 
rangement of these cells are shown in figs. 16, 17, and 18. The 
