200 



THE ALG& 



FIG. 191. Botrydiurn and 

 Protosiphin 



These forms of the siphon algae 

 are almost indistinguisha- 

 ble in the vegetative condi- 

 tion. The plants are little 

 green globes, somewhat 

 larger than pin heads, at- 

 tached to the surface of mud 

 and wet earth by a branch- 

 ing system of filaments 

 (rhizoids). These single 

 coenocy tic cells are therefore 

 differentiated into a green 

 part above ground, exposed 

 to the air and sunlight, and 

 a colorless underground por- 

 tion in contact with mois- 

 ture. A single cell may thus 

 show the same relation of 

 parts as a complex plant with 

 aerial stems and leaves and 

 (i subterranean root system. 



at the end of a branch, which is gener- 

 ally bent like a crook (Fig. 190, A, B, a), 

 and discharges a very large number of 

 small, two-ciliate sperms (Fig. 190, C). 

 The sperms enter the oogoniuin through 

 a pore in the beak-like extension at one 

 side, and one of them, fusing with the 

 egg, fertilizes it. The fertilized egg sur- 

 rounds itself at once with a heavy wall, 

 becoming an oospore (Fig. 190, A, os), 

 which is a resting spore in this form, as 

 in Volvox, (Edogonium, Coleochcete, etc. 



Vaiicheria has been made the sub- 

 ject of some interesting experimental 

 studies on the conditions which deter- 

 mine the formation of zoospores and 

 sexual organs respectively. Zoospores 

 are generally developed at once, follow- 

 ing some marked change in the exter- 

 nal conditions, as in the character of 

 the water, or an increase in light expo- 

 sure, or a rise in temperature. Sexual 

 organs are formed when plants are cul- 

 tivated in a 2-5 per cent solution of 

 cane sugar at a fairly high temperature 

 (50-68 F. ; 10-20 C.), and in the 

 presence of light. The conditions in 

 Vaucheria probably illustrate very well 

 the kinds of influences which cause an 

 alga to begin its various forms of fruc- 

 tifications, but very few algse have been 

 studied in detail 



229. The ccenocyte. The large, many- 

 nucleate structures called ccenocytes, 

 so well illustrated by the siphon algae 



