Ch. X] 



THE TUBULAR ALG^ 



431 



all of the nuclei except one having passed meantime into the 

 main filament. Close alongside, and in some kinds from the 

 same stalk, is formed a smaller tubular branch from which is 

 cut off a terminal antheridium, wherein are developed numer- 

 ous ciliate sperm cells. The one of these which first gains 

 entrance to the oogonium, through a special opening which 

 appears, fertilizes the egg cell, wherefrom there develops 

 a thick-walled resting 

 oospore, which later 

 germinates to a new 

 filament. These oogo- 

 nia and antheridia are 

 of special interest as 

 the first structures met 

 in the ascending scale 

 which are devoted spe- 

 cially to the formation 

 and fusion of the sexual 

 cells; for in all lower 

 forms the cells which 

 produce the reproduc- 

 tive bodies are only 

 transformed vegetative 

 cells. Another common 

 form is Botrydium (Fig. 

 299), which grows in 

 masses upon wet, clayey 

 earth, especially when drying from a flooded condition. The 

 single plants, each about the size of a pin head, are balloon 

 shaped, the lower part being differentiated into colorless 

 rhizoids which enter the earth and there absorb water and 

 mineral salts for the green part in the light; and herein is 

 anticipated the physiological differentiation into root and 

 shoot prevailing in the higher plants. It reproduces asexu- 

 ally by zoospores, which are formed in great numbers, and also 

 sexually by conjugation of small biciliate isogametes. 



Fig. 299. — Botrydium granulatum. 



Left, an adult individual ; next, individual 

 producing and expelling zoospores ; next, be- 

 low, young plant germinating from a zoospore ; 

 all X 25. Above, right, a zoospore, with 

 two chloroplasts ; X 500. (After L. Kny.) 



