28 



PLANT STRUCTURES 



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

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



-A 



B 



FIG. 11. Vaucheria sessilis, a Siphon form, show- 

 ing a portion of the ccenocytic body, an an- 

 theridial branch (A) with an empty anthe- 

 ridinm (a) at its tip, and an oogouium (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 egg (Fig. 11, B}. The an- 

 theridiuin is a much smaller cell, 

 within which numerous very small 

 sperms are formed (Fig. 11, A, a). 

 The sperms are discharged, swarm 

 about the oogonium, and finally 

 one passes through the beak 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. 12). 



26. 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. 12. Botrydium, one of the 

 Siphon forms of green algae, 

 the whole body containing 

 one continuous cavity, with 

 a bulbous, chlorophyll-con- 

 taining portion, and root- 

 like branches which pene- 

 trate the mud in which 

 the plant grows. CALD- 

 WELL. 



