16 MORPHOLOGY 
Structure. The structure of the protoplast olChlamydomonas(fig. 21) 
may be taken as representative of the whole group. There is usually a 
single, large, cup-shaped chloroplast at the larger end of the protoplast, 
in which is embedded a large protein body (pyrenoid). In the cup of 
the chloroplast the nucleus is found; near the base of the cilia are two 
contractile vacuoles; at the forward end a red pigment spot ("eye spot") 
is observed ; and two long apical cilia complete the equipment. The 
cells are very active and their motion is influenced by light, to which the 
"eye spot" is supposed (without adequate ground) to be very sensitive. 
In some forms a red pigment appears so abundantly as to give a reddish 
hue, giving rise to the accounts of "red pools" and "red snow." Under 
certain conditions the cell may drop its cilia and become quiescent, and 
this temporary loss of motility in the vegetative cells of Volvocales 
becomes the permanent condition in higher forms. 
Reproduction. In this quiescent stage, the protoplast may divide 
into several new cells, which escape as new and active individuals. 
These daughter cells, formed within the old mother cell, are called 
zoospores (swimming spores, swarm spores), but they are also the adult 
form of the plant. Therefore, the ordinary vegetative cells of Volvo- 
cales are like the zoospores of the higher forms, in which the vegetative 
cells and zoospores are quite distinct. 
Certain cells form more numerous and smaller zoospore-like cells, 
which escape, swim freely, and fuse in pairs to form new cells (figs. 24- 
27). This is a sexual process, and therefore these pairing cells are 
called gametes (sexual cells). Since the pairing gametes are alike, the 
condition is called isogamy, and the plants are said to be isogamous. It 
is evident that the gametes are related to the zoospores, and it is thought 
that they are only modified zoospores. The origin of gametes is the 
origin of sex, and isogamy is the simplest form of sexuality. This fusion 
of two gametes to form a single cell is the act of fertilization, but to 
distinguish it from the higher forms of fertilization it has become cus- 
tomary to call the fusion of similar gametes conjugation. The cell 
resulting from conjugation is a zygospore (or zygote), spore being the 
general name of a cell set apart for reproduction, and the prefix in this 
case indicating that the spore has been formed by conjugation. In 
isogamous plants, therefore, the gametes conjugate and produce a 
zygospore. In general the zygospore is a resting cell, being formed at 
the inception of unfavorable conditions, having a heavy wall, and 
starting new generations upon the return of favorable conditions. In 
