THALLOPHYTES 
the case of Sphaerella, when the zygospore germinates, the protoplast 
divides, forming two or four cells, which escape as free-swimming cells. 
It should be remarked that reproduction by zoospores (an asexual 
method) results in multiplying plants during the growing season; while 
reproduction by zygospores (the sexual method) is connected with the 
formation of a protected cell which endures unfavorable conditions. 
Pandorina. Among Volvocales, as among the previously described 
groups, there is a prevailing tendency to colony formation, which fi- 
nally reaches an extreme 
expression. Pandorina 
illustrates a simple colony, 
which is composed usually 
of sixteen similar cells 
held together by a gelati- 
nous matrix (fig. 23). 
The protoplast of any cell 
of the colony may divide 
into sixteen daughter cells, 
which form a new colony 
that escapes from the 
mother cell and from the 
mother colony. There is 
also sexual reproduction pics. 33-27. Pandorina: 23, the free-swimming 
as described above, the colony; 24, a gamete; 25, two gametes beginning to 
gametes being produced ^se; 26, fusion of gametes almost complete; 37 , the 
zygospore. After PRINGSHEIM. 
just as are the daughter 
colonies. The pairing gametes are similar, and therefore Pandorina 
is isogamous and the sexually formed spore is a zygospore (figs. 
24-27). 
Eudorina. In this form there is a larger colony or cell family. New 
families are formed as described for Pandorina, but sexual reproduc- 
tion is different (fig. 28). Certain cells of the colony, not much different 
from the vegetative cells, assume the function of eggs. Other cells 
divide to form groups of sperms, which hang together in a plate, but 
finally break up and fuse with the eggs. In this case the pairing gametes 
(egg and sperm) have become very dissimilar, and hence the condition 
is called heterogamy, the plant being heterogamous. The larger gamete, 
often called the oosphere, is better called the egg; and the smaller one, 
often called antherozoid or spermatozoid, is better called the sperm. The 
