CRYPTOGAMS 



355 



the protoplast by a gelatinous layer except at the anterior end. Haematococcus 

 pluvialis 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 ( 23 ), 

 there is in contrast to the other species a marked differentiation 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. 



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

 and saprophytic form (Fig. 282, 2). 



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

 or coenobia. In Volvox ( 24 ) (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 enlarge- 

 ment of single cells of the colony; they are large, green, non- motile cells 

 surrounded by a mucilaginous wall. The small spermatozoids are elongated 

 bodies of a bright yellow colour, provided with two cilia at 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. 



Order 2. Protococcales. These are unicellular green Algre, 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 contains a nucleus 

 and only one chloroplast. Reproduc- 

 tion is by means of zoospores, in place 

 of which in many genera non-ciliated 

 aplanospores are found. Sexual repro- 

 duction, when present, takes the form of 

 conjugation of similar gametes. 



The simplest forms belong to the 

 genera Chlorococcum (including Cysto- 

 coccus) and Chlorella C 25 ' 26 ). The cells 

 of the former are spherical, and occur 

 in fresh water and also on damp sub- 

 strata ; 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. 285) ; under certain conditions these are replaced by aplanospores 

 without cilia. Chlorella milgaris (Fig. 286) is a wide - spread Alga, the small 



2 A 1 



FIG. 2S5. Chlorococcum (Chlorosphaera) limicola. 1. 

 Vegetative cell and cell divided into 8 zoospores ; 

 2, free zoospores ; 3, zoospores after they have 

 formed cell-walls. (After BEYERINCK, from 

 OLTMANNS' Algae.) 



