FACTS AND OBSERVATIONS. 
935 
outer layer or skin be stripped from the surface of the green- 
coloured parts of plants, and examined under a low power of 
the microscope, the stomata, or breathing pores, will appear 
as green specks in the otherwise colourless membrane. Their 
object is to open'and close communication between the 
intercellular space always existing between the individual 
cells and the outer atmosphere. The sausage-shaped cells 
constituting the essential part of the organ are called the pore 
cells. They have the power of separating from each other in 
the middle, thus opening a free way for the air to the interior 
tissues; or in certain conditions of light and moisture they 
approach each other so as to narrow or entirely close the slit 
between them. They are filled with protoplasm, chorophyl 
and starch granules, while all other cells of the outer surface 
are filled only with air and water. Apparently with the 
object of placing these pore cells as free as possible from all 
constraint or pressure, so that they may correspond sensi¬ 
tively to all the changes in the atmosphere, they are at times 
situated on a level with the epidermis cells, sometimes raised 
above, at others sunk beneath this level. If the epidermis 
cell-walls are thin and flexible, the stomata will generally be 
found in the same surface with them; but when the epidermis 
walls are thick and stiff, the stomata will generally be found 
sunk deep under the surface, or raised above it, or surrounded 
by a ring of smaller cells with thinner walls than the remain¬ 
ing epidermis cells. Immediately under the stomata are 
empty spaces, of irregular form and varying size, called 
breathing-rooms. They are in connection with, and form a 
part of the intercellular space which ramifies through the 
entire structure of most tissues. It is an interesting question 
in what way the stomata have been formed. Were the pore 
cells at first a pair of ordinary cells, which have gradually 
changed their form and contents until endowed with all the 
peculiar properties of their natural state? Or were they 
always existent in their peculiarities, only smaller as the leaf 
was younger ? Or have they grown out of a single cell by 
the process of subdivision and after-growth ? Do they belong 
to the epidermis, or to the chlorophyl-bearing tissues be¬ 
neath ?— Ibid. 
