DESCRIPTION OF PLATES. 139 
tially what occurs in sexual reproduction among 
plants and animals higher in the scale, viz., the 
coalescence of two cells to form a new individual. 
This process is called conjugation, and it is de- 
scribed as follows by Dr. Carpenter, as it occurs 
in Palmogloea : — 
“The conjugating process commences by the 
putting forth of protrusions from the boundaries of 
two adjacent cells, which meet, fuse together, and 
form a connecting bridge between their cavities. 
The fusion extends before long through a large 
part of the contiguous sides of the two cells; and 
at last becomes so complete that the combined 
mass shows no trace of its double origin. It soon 
forms for itself a firm cellulose envelope, which 
bursts when the ‘ zygospore’ is wetted; and the con- 
tained cell begins life as a new generation, speedily 
multiplying, like the former ones, by binary 
division.” 
This evidently corresponds with what occurs 
in sexual reproduction, which is the only mode of 
increase among the highest plants and animals. 
In the unicellular alga, however, the two cells 
are apparently alike; while in flowering plants and 
in animals they differ in appearance. 
Conjugation in phenogamous plants occurs by 
the falling of the pollen —the male element — 
upon the pistil, and the extension of a slender 
tube from the pollen-grain down to the germ- 
cell — female element — contained in the ovule. 
Through this tube the contents of the pollen-grain 
are discharged, and we thus have a commingling 
