SCIENTIFIC SUMMARY. 
311 
Crit. Ic. Muse. FI. Dan.,” tliat this species is in the St. Petersburg herbarium 
named tmellum in Ehrhart's own handwriting ; this, however, without 
description, might not be allowed to stand, but the same species received 
the same name from Persoon, as proved by a specimen from him, preserved 
in Swartz’s herbarium ; and a description is given by Bridel in his “ Mantissa 
Muse.” (1819), the leaves indeed being described as recurved at the point, 
which might perhaps refer to them in a dry state. Bridel also admits 
S. molhiscum into the Bryol. Univ., but he only copied the description of 
Bruch (1825), ^without having seen a specimen. Dr. Braithwaite, there- 
fore, has no hesitation in adopting the name first given to the species. 
The Breathing Pores of Leaves . — A good popular paper on this subject is 
that which Prof. T. D. Biscoe read before the Troy Scientific Association, 
and published in the “American Naturalist,” March, 1872. If, he says, the 
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 sensitively 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 remaining epidermis cells. Immediately under the 
stomata are empty spaces, of irregular form and varying size, called breath- 
ing-rooms. They are in connection with, and form a part of the intercel- 
lular 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 pro- 
perties 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 beneath ? 
