THE ALGAE 



59 



In sexual reproduction the active cell comes to rest and the flagella are 

 withdrawn. The contents then divide up into either thirty-two or sixty-four 

 gametes. These also resemble the parent cells, except that they are much 

 smaller and are devoid of a cell wall. Under suitable conditions they are 

 liberated by the bursting of the parent cell. They then fuse in pairs, each pair 

 forming a zygote. In fusion the two anterior ends come together and the 

 contents of the one gamete pass into the other. The zygote then secretes a 

 thick wall and thus becomes a cyst. There is usually no difference in the 

 size or structure of the gametes, and hence they are said to be isogamous. 

 In some species, however, the gametes are alike in structure but differ in size, 

 and they are then termed heterogamous or anisogamous. 



In Chlamydomonas hraunii the gametes are dissimilar in size. 

 Macrogametes are produced in fours from the parent cell, which they 

 resemble ; while the microgametes, which are formed in eights, are only 

 about half the size and more elongated. The macrogamete soon comes 

 to rest, withdraws its flagella and is sought after by the active microgamete. 

 Both possess a cell wall, and 

 after union by their apical ends 

 the membranes coalesce and the 

 contents of the microgamete 

 pass over into the macrogamete. 

 During this process the flagella 

 disappear and a close-fitting ^yg*^, 

 membrane is secreted by the 

 zygote. In this species, there- 

 fore, Chlamydomonas approaches 

 an oogamous condition. 



The life-history of Chlamy- 

 domonas may be represented in 

 the form of a diagram (Fig. 34) 

 showing the succession of stages 

 which occur both in sexual and 

 asexual reproduction. 



32-64 gametes 



4-16 zoospores 



^^^Motile Cell- 

 FiG. 34. — Life-cycle of Chlamydomonas. 



Haematococcus (Sphaerella) 



The active cell in this organism differs from that of Chlamydomonas 

 (Fig. 35) chiefly in the separation of the protoplast from the cell wall. The 

 cell wall itself is well defined but thin and is composed of cellulose. It is 

 separated from the cvtoplasm by a thick mucilaginous inner wall which is 

 penetrated by fine protoplasmic threads. Embedded in the central cytoplasm 

 is a large hemispherical chloroplast which is hollowed out and resembles 

 that in Chlamydomonas. In fact the structure of the whole protoplast is sub- 

 stantially similar to that of Chlamydomonas, except that there are numerous 

 contractile vacuoles and several pyrenoids. 



Haematococcus obtains its name from the fact that the cell is often red m 



