358 



BOTANY 



PART II 



jihivialis occurs commonly in puddles of rain-water, and, like Chlamydomonas 

 nivalis, which gives rise to "red-snow" and occurs on snow in the Alps, etc., 

 is characterised by the presence of a red pigment (haematochrome) in the cells. 

 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 Chlamydomonas coccifera, according to Goroschankin (^), 

 there is in contrast to the other species a marked ditferentiation in the sexual 

 cells. Single cells become transformed into large, non-ciliated, female gametes or 

 egg-cells ; others divide and each gives rise to 16 small, biciliate male gametes. 

 The transition to oogamy thus occurs in this group even among the isolated 

 unicellular forms. 



Pohjtoma urdla, which resembles Chlamydomonas in structure, is a colourless 

 and saprophytic form (Fig. 27-3, 2). 



The biciliate cells of Pandorina, Eudorina, Volvox, etc., are united in colonies 

 or coenobia. In Volvox (^^) (Fig. 284), which may be regarded as the highest form 

 in the order, the free-swimming colonies have the shape of a hollow sphere. The 

 component protoplasts are connected by fine processes, so that the organism must 

 be regarded as constituting a single individual. The sexual cells are differentiated 

 into ova and spermatozoids. The egg-cells arise by the 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 

 ecrs-cell is transformed into the thick- 



The vegeta- 



Fifi. 276. — Chlorococcum{Chlorosphaera)li'micola. 1, 

 Vegetative cell and cell divirlod into S zoospores ; 

 2, free zoospores ; 

 formed cell walls 



Oltmanns' Algae.) 



walled, resting oospore 

 zoospores after they have tive reproduction of Volvox takes place 

 (After Beyerinck, fron, ,^y ^j^^ 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. 



Order 2. Protococcales. — These are unicellular green Algae, or their cells are 

 united in colonies of various form ; the vegetative cells have no cilia, and the cell 

 or colony is consequently non-motile. Usually each cell contaius a nucleus and 

 only one chloroplast. Reproduction is by means of zoospores, in place of which 

 in many genera non-ciliated aplanospores are found. Sexual reproduction, when 

 present, takes tlie form of conjugation of similar gametes. 



The simplest forms belong to the genera Chlorococcum and Chlorella (^"^ ^'). 

 The cells of tlie former are spherical, and occur in fresh water and also on damp 

 substrata ; they frequently take part in the composition of Lichen thalli. Asexual 

 reproduction is by the production from a cell of a number of biciliate zoospores 

 (Fig. 276) ; under certain conditions these are replaced by aplanospores without 

 cilia. Chlorella vulgaris (Fig. 277) is a wide-spread Alga, tlie small cells of which 

 often live symbiotically in the protoplasts of lower animals (Infusoriae, Hydra, 



