DESCRIPTION OF PLATES. 167 
For information in regard to the origin, struct- 
ure, and functions of the blood-corpuscles, the 
reader is referred to standard works upon physi- 
ology and upon histology. Foster’s “ Physiology ” 
may be especially recommended. 
PoLLeN Grains. (PuatTe VII.) 
Pollen grains are living cells detached from 
the male organs of flowering plants, which are 
necessary for the fertilization of the ovule, which 
subsequently becomes a seed containing an em- 
bryo-plant. Like the spores of Fungi, they con- 
tain a portion of the protoplasm of the parent 
plant enclosed in a cell-wall, and, like these, 
they send out a slender filament when they are 
deposited in a locality suited for their germina- 
tion. But, unlike these, they are incapable by 
themselves of producing a new plant, and they 
are even more particular than are the microscopic 
Fungi as regards the substratum upon which 
their germination may take place. This is lim- 
ited to a great extent to the surface of the 
stigma of a plant of the species to which they 
belong. When so located, the pollen tube pene- 
trates between the cells of the stigma, and, 
nourished by the juices of the female organ, 
extends through the whole length of the style 
to the ovule,— often a distance of an inch or 
more. Through this tube the fluid protoplasm, 
which contains numerous granules, passes, to be 
commingled with the protoplasm of the germ-cell. 
