DIV. I 



THALLOPHYTA 



399 



Reproduction is both asexual, by swarm-spores, 2-8-16 of which are formed in a 

 mother cell and are set free by rupture of the membrane, and sexual ; the sexual 

 reproduction is by conjugation of similar, small, biciliate gametes formed in large 

 numbers (to 64) in a 

 mother cell, and uniting 

 in pairs by their anterior 

 ends to form a zygote. In 

 Chid. ni ydomonas cocci- 

 feru, according to GORO- 

 SCHA.NKIN ( M ), there is 

 in contrast to the other 

 species a marked differen- 

 tiation in the sexual 

 cells. Single cells become 

 transformed into large, 

 non-ciliated, female 

 gametes or egg - cells ; 

 others divide and each 



i : A, swarming cell ; 



FIG. 332. .4, B, Haematococcus pluvialis (x 



B, formation of swarm -spores. C-G, Haematococcus Biitschlii : 



C, formation of gametes (x400); D, gamete; E, conjugation of 

 two gametes ; F, G, zygotes (x 800). (C-G after BLOCHMANN.) 



gives rise to 16 small, 



biciliate male gametes. The transition to oogamy thus occurs in this group even 



among the isolated unicellular forms. 



Polytoma uvella, which resembles Chlamydomonas in structure, is a colourless 



and saprophytic form (Fig. 333, 2). 



Under certain conditions some Chlamydomonads lose their cilia and the cells 



enclosed in swollen gelatinous walls 

 undergo divisions and form colonies. 

 This is termed the Palmella stage. The 

 cells can again become motile under 

 favourable conditions. 



The biciliate cells of Pandorina, 

 Eiidorina, Volvox, etc., are united in 

 colonies or coenobia. In Volvox (Fig. 

 334), which may be regarded as the 

 highest form in the order, the free- 

 swimming colonies have the shape of a 

 hollow sphere. The component proto- 

 plasts are connected by fine processes, 

 so that the organism must be regarded 

 as constituting a single individual. 



The sexual cells are differentiated into 



j .j m, ,, 



" d ^matozoids. The egg-cells 



anse b Y tne enlargement of single cells 

 of the colony ; they are large, green, 

 non-motile cells surrounded by a muci- 

 laginous wall. The small spermatozoids 



are elongated bodies of a bright yellow colour, provided with two cilia attached 

 laterally below the colourless anterior end ; they arise by the division of a cell 

 of the colony into numerous daughter cells. After fusing with a spermatozoid 

 within the cavity of the colony the egg-cell is transformed into the thick-walled, 

 resting oospore. The vegetative reproduction of Volvox takes place by the division 

 of single cells of the colony to form a new daughter colony ; this corresponds 

 to the formation of swarm-spores in other genera. Eudorina is also oogamous. 



CUT 



FIG. 333. 1, Chlamydomonas angulosa (after 

 DILL); g, cilia; ,, vacuole ; *, nucleus; ckr, 

 chromatophores ; py, pyrenoid ; a, eye-spot. 

 2, Polytoma uvella (after DANOEARD). (From 

 OLTMANSS' Algae.) 



