244 



PLANT STUDIES 



and cut off from the general cavity by partition walls (Fig. 

 210). The oogonium becomes a globular cell, which usually 



Fig. 210. Vaucheria sessilis, a Siphon form, 

 showing a portion of the ccenocytic body, an 

 antheridial branch (.4) 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 beak for 

 the entrance of the sperms, and 

 organizes within itself a single 

 large Qgg (Fig. 210, B). The an- 

 theridium is a much smaller cell, 

 within which numerous very small 

 sperms are formed (Fig. 210, .1, a). 

 The sperms are discharged, swarm 

 about the oogonium, and finally 

 one passes through the beak and 

 fuses with the Qgg^ 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 BotrycUum 

 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. Botrydhim, 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. 



