1877.] Reproduction in Fresh- Water Alge. -515 
present. The general form of a desmid is usually very symmet- 
rical, it being composed of two parts, of which the one is the ex- 
act counterpart of the other. (Figure 86, a and b.) The asexual 
form of reproduction is a cell multiplication by division, but differs 
somewhat from the cases already mentioned. When two des- 
mids are to be made out of one, the first process is the elongation 
of the neck or part by which the two halves join (e). ‘ By this 
time a new wall has formed inside each half of the isthmus and 
stretches also across its cavity, forming with its fellow a double 
partition wall separating the two halves of the old frond. Rapid 
growth of the newly formed parts now takes place, the central 
ends become more and more bulging as they enlarge, and in a 
little time two miniature lobules have shaped themselves at the 
position of the old isthmus [d]. At last, the parts thus formed 
having assumed the shape and appearance of the original lobules, 
the two fronds which have been developed out of one separate 
mostly before the new semicells [e] have acquired their full 
size.” 1 This is much the most common form of reproduction, 
the other being the sexual method, and in one sense a process 
directly opposite to the one just described, it being the union of 
two cells to form one. When it is to take place, the outer walls 
of two desmids lying near each other burst open and the con- 
tents of each cell, with its thin inner wall, protrude; these finally 
blend, and the contents of the two cells flow into’ one mass (i). 
This new cell thus produced soon takes on a thick outer coat 
which is frequently ornamented with spines and other markings, 
and the ultimate result of the whole process is a spore. 
In the Desmids the two cells which unite exhibit no sexual dif- 
ferentiation, and botanists have given to all such cases the name 
of conjugation in distinction from fertilization, where the male 
and female organs are apparent. But what becomes of the prod- 
uct of the two conjugated cells? It isa resting spore provided 
with a thick outer coat, and may sink to the bottom of the pond, 
where it remains through the winter. When the time for ger- 
mination arrives its contents divide into a number of masses, 
each of which becomes a young desmid surrounded by a cell- 
Wall of its own, and is turned out to take care of itself when the 
Wall of the spore is broken away. 
Thus far we have looked only at plants which are distinctly 
Unicellular. Now we come to those that are made up of a num- 
ber of single-celled individuals capable of existing alone, but usu- 
1y arranged end to end in the form of filaments. 
1 Wood’s Fresh-W ater Algæ. 
