sEcr. I 



CRYPTOGAMS 



347 







two equivalent gametes to form a zygote or zygospore. Asexual 

 reproduction by means of swarm-spores is Avanting in both classes, 

 and since the gametes also are unprovided with cilia these forms are 

 also classed as Acontae. The cells of the Conjugatae, which increase 

 in number by cell-division, 

 are uninuclear, and differ 

 from those of the Diatoms 

 in having a cell wall which 

 is not silicified and in the 

 presence of large green 

 chloroplasts of compli- 

 cated structure. 



1. The Mesotaeniaceae, 

 the simplest of the unicelhilar 

 Conjugatae, include only a 

 few genera. They are dis- 

 tinguished from the following 

 order by the cell wall of the 

 shortly cylindrical cells not 

 being formed of two halves. 

 The mode of conjugation pre- 

 sents some differences. In 

 Spirotaenia (Fig. 260) the pro- 

 toplasts of the conjugating 

 cells first divide and the 

 daughter cells unite in pairs 

 to form the zygotes ; in 3Ieso- 

 taenium and Cylindrocystis 

 the protoplasts conjugate with- 

 out a preliminary cell- divi- 

 sion. Similar differences are 

 shown by the Diatomeae. The 

 zygote on gerjuination divides 

 into four new individuals. 



2. The Desmidiaceae are 

 unicellular or their cells are 

 united in rows ; they are of 

 great beauty and, like the 

 Diatoms, exhibit a great 

 variety of form. Their cells 

 are composed of two symmetri- 

 cal halves, separated, as a rule, from each other by a constriction. Each half 

 contains a large, radiate chromatophore or a chromatophore composed of a number 

 of plates. Within the chromatophores are disposed several pyrenoids, while the 

 nucleus lies in the centre of the cell in the constriction. The cells themselves 

 display a great diversity of form and external configuration, being sometimes 

 rounded (e.j/. Cosinarium, Fig. 261, A, B), sometimes stellate {Micr aster ias, Fig. 261, 

 D). The cell walls, which, as in the Diatoms, consist of separate halves, are 

 frequently beset with wart- or horn -like protuberances and often provided with 





\ 



B 



m 



i"^> 



Fiii. :2ijl. — A, Cosmarium, codatum, dividing. B, C, Cosmarium 

 botrytis. C'l, two cells at right angles preparing for con- 

 jugation — the lower cell shows the conjugation canal ; C'2, 

 gametes fused into the young zygote ; C3, mature zygote ; 

 D, Micrasterias crux melitensis. (After Ralfs ; C'l, C'2, 

 after De Bary.) 



