CRYPTOGAMS 



319 



TANGIA. Cells in which spermatozoids take their origin are termed 

 ANTHERIDIA; those giving rise to egg-cells, OOGONIA. If the sexual 

 form be derived from an asexual form of reproduction, all these organs, 

 as well as those similarly named in the other classes of the Thallophytes, 

 must be regarded as homologous. 



The Conjugatae and Charaeeae, as well as the three orders of the Chlorophyceae, 

 also possess green ehromatophores, and hence the designation Green Algae, in its 

 widest, unrestricted sense, is also applicable to them. The Conjugatae, however, 

 are sharply characterised by their peculiar manner of sexual reproduction. The 

 Charaeeae also form a distinct group, and are marked off from the Chlorophyceae 

 by the more highly advanced segmentation of their thallus and the more compli- 

 cated structure of the female sexual organs and of the antheridia, both of which 

 are enclosed within special enveloping receptacles, while the antheridia and oogonia 

 of the Chlorophyceae are always devoid of any external covering of sheathing 

 sterile cells. 



Order 1. Protoeoeeoideae 



The Protoeoeeoideae include only unicellular Algae, whose cells lead a 

 separate existence, or are united into cell families with a definite or in- 

 definite order of arrangement. They occur, for the most part, as freely- 

 swimming, fresh-water forms, but are also found in damp places. The 

 cells are uninuclear, and contain one or more ehromatophores. In the 

 simpler forms multiplication takes place vegetatively by cell division ; 

 but, in most cases, asexual swarm-spores, provided with two cilia, are 

 produced. Sexual reproduction, which does not occur in all genera, 

 is effected by the conjugation of two exactly similar planogametes 

 which fuse into a zygospore or zygote. The fertilisation of an 

 egg by a motile spermatozoid is only known to take place in the case 

 of Eudorina and Volvox. 



Many of the Protoeoeeoideae are polymorphous, and assume, according to the 

 season of the year and the conditions of their environment, different external forms 

 corresponding to different stages in their development. 



Scenedesmus acutus, a polymorphous free-swimming form, very common every- 

 where in water, is gener- 

 ally found in small cell- 

 families, consisting of 

 four spindle-shaped cells 

 lying close together (Fig. 

 239, i, k). Under certain 

 conditions, however, this 

 Alga passes into the Pal- 

 mella stage, and it then 

 appears as spherical cells, 

 multiplying by cell divi- 

 sion (a, b). Each of these 

 cells may again divide 

 into four spindle-shaped 

 cells, which, after escaping from the mother cell, either remain isolated (c, d, e) 



Scenedesmus aeittus. Different stages of development. 

 (After Chodat.) 



