230 PLANT STUDIES 
old cell, is often called internal division, to distinguish it 
from fission, which involves the wall of the old cell, and is 
the ordinary method of cell division in nutritive cells. 
If the mother cell which produces the spores is different 
from the other cells of the plant body it is called the sporan- 
gium, which means " spore vessel." Often a cell is nutri- 
tive for a time and afterward becomes a mother cell, in 
which case it is said to function as a sporangium. The wall 
of a sporangium usually opens, and the spores are dis- 
charged, thus being free to produce new plants. Various 
names have been given to asexual spores to indicate certain 
peculiarities. As Algae are mostly surrounded by water, 
the characteristic asexual spore in the group is one that 
can swim by means of minute hair-like processes or cilia, 
which have the power of lashing the water (Fig. 206, C). 
These ciliated spores are known as zoospores, or " animal- 
like spores," referring to their power of locomotion ; some- 
times they are called swimming spores, or swarm spores. It 
must be remembered that all of these terms refer to the 
same thing, a swimming asexual spore. 
This method of reproduction may be indicated by a for- 
mula as follows : P o P o P o P, which indi- 
cates that new plants are not produced directly from the 
old ones, as in vegetative multiplication, but that between 
the successive generations there is the asexual spore. 
Sexual spores. These cells are formed by cell union, 
two cells fusing together to form the spore. This process 
of forming a spore by the fusion of two cells is called the 
sexual process, and the two special cells (sexual cells) thus 
used are known as gametes (Fig. 205, C, d, e). It must be 
noticed that gametes are not spores, for they are not able 
alone to produce a new plant ; it is only after two of them 
have fused and formed a new cell, the spore, that a plant 
can be produced. The spore thus formed does not differ 
in its power from the asexual spore, but it differs very 
much in its method of origin. 
