214 PLANT LIFE 
place to glance at a few examples, in order to 
gain some knowledge of the general character 
of the sexual process itself so far as we at 
present understand it. At the same time, we 
shall be in a better position to appreciate the 
bearings of its elaboration on the evolution 
of the series of higher plants. 
If we once more take as our starting-point 
a relatively simple unicellular plant such as 
Chlamydomonas, we find that under certain 
conditions it continues to grow and to multiply 
itself vegetatively (see p. 15). After a time, 
however, and under certain altered nutritive 
conditions, sexual reproduction sets in (Fig. 
24). The young individuals which have been 
recently liberated from parent cells, after 
swimming about for a while, undergo a change. 
The living protoplasmic body slips out of 
the cellulose skin, and swims as a naked cell 
in the water. Very soon these cells are 
observed to approach one another in pairs. 
Two individuals become attached, and then 
gradually coalesce. The cilia disappear, and 
the now motionless zygote becomes spherical 
and surrounds itself with a new cell wall. 
Chemical changes continue to go on within 
its body, for the chlorophyll loses its green 
colour and gives way to a red pigment. 
Later on, and after a longer or shorter period 
of rest, the green colour returns, the cell 
reawakens to vegetative activity, its contents 
divide, and new chlamydomonas individuals 
are produced. 
