244 
PLANT STUDIES 
and cut off from the general cavity by partition walls (Fig. 
210). The oogonium becomes a globular cell, which usually 
B 
FIG. 210. Vaucheria sessilis, a Siphon form, 
showing a portion of the ccenocytic body, an 
antheridial branch (A) with an empty anthe- 
ridium (a) at its tip ; and an oogonium (B) 
containing an oospore (c) and showing the 
opening (/) through which the sperms passed 
to reach the egg. CALDWELL. 
develops a perforated break for 
the entrance of the sperms, and 
organizes within itself a single 
large egg (Fig. 210, B). The an- 
theridium is a much smaller cell, 
within which numerous very small 
sperms are formed (Fig. 210, A, a). 
The sperms are discharged, swarm 
about the oogonium, and finally 
one passes through the break and 
fuses with the egg, the result be- 
ing an oospore. The oospore or- 
ganizes a thick wall and becomes 
a resting spore. 
It is evident that Vaucheria is heterogamous, but all 
the other Siphon forms are isogamous, of which Botrydium 
may be taken as an illustration (Fig. 211). 
167. Spirogyra. This is one of the commonest of the 
" pond scums," occurring in slippery and often frothy 
masses of delicate filaments floating in still water or about 
FIG. 211. Botrydium, one of 
the Siphon forms of green 
algae, the whole body con- 
taining one continuous cav- 
ity, with a bulbous, chloro- 
phyll - containing portion, 
and root - like branches 
which penetrate the mud 
in which the plant grows. 
CALDWELL. 
