674 Our: Fresh-Water Alge. 
find it in the presence respectively of antheridia and archegonia. 
In the violet batrachosperms of fresh water these organs are pro- 
duced on separate individuals. The antheridia contain small motile 
bodies, antherozoids, analogous to the pollen contained in the 
anthers of flowering-plants, and to the spermatozoids of animal life. 
These antherozoids find their way upon the other plant to where a 
long hair-like tube (the trichogyne) opens, through which their 
fertilizing influence reaches the protoplasm mass in the bulbous 
base of the tube (the archegonium or carpogonium). The proto- 
plasm on fertilization swells, divides, usually forms new cells around 
it, as if walling itself in, and then a series of new cells within, 
many of which become spores, the whole fruit so formed becoming 
as full of spores as a stramonium pod of seeds, and generally 
resembling the latter in their position as well. 
There is great variety of form among the alge of fresh water, 
even among the unicellular species. It might be thought that these 
species, where the whole plant is composed of but a single cell, 
would present little variety ; especially when it is considered that 
such simple cells commonly float loosely in the water, and in situa- 
tions enabling the supposed normal spheroidal cell-form to develop 
itself, free from the influences of crowding or lateral stimuli. But 
not so simple is the plan of nature, and a great range of shape exists 
among the single-celled algæ, from the spherical of the common 
protococcus of our trees and walls to the bur-like spiny Polyedrium. 
For instance, one Rhaphidium is crescent-shaped, another needle- 
shaped, another unicellular alge is shaped somewhat like the letter 
S, another like a J, anothera C. The Botrydium is balloon-shaped, 
the Chytridium often urn-shaped, others appear as little discs, others 
ellipses, others cubical or pentagonal. When associated in masse, 
pressure and the exigencies of growth change the shape of those 
naturally circular into irregular polygons. Some species of Ophio- 
cytium grow into curious coils; some Polyedriums are exact : 
triangles, others take the form of a Greek cross. Extend our view 
to the desmids and diatoms, which are also of the unicellular alge 
of fresh-water, and the number of cut and fantastic forms which a 
plant of a single cell may present, becomes indefinitely increased. 
The larger number of species of the fresh-water alge are, hows 
ever, of more than onecell. Of these multicellular alge some grow 
