DIV. 



THALLOPHYTA 



393 



plasts of complicated structure. Asexual reproduction by swarm-spores 

 is wanting in them as in the Diatoms, with which they also show 

 points of agreement in their sexual reproduction. This consists in the 

 conjugation of two equivalent non-ciliated gametes to form a zygote or 

 zygospore. 



The Conjugatae and Diatomeae were formerly united in one group called the 

 Zygophyceae, or on account of the non-ciliated gametes, the Acontae. This is no 

 longer regarded as a natural grouping. It has been seen above that some Diatoms 

 have ciliated gametes. The reduction divi- 

 sion in the Conjugatae occurs after conju- 

 gation in the germinating zygote, while in 

 the Pennatae it takes place at the formation 

 of the gametes. Tfee two groups have 

 evidently originated independently from 

 the Flagellatae. 



1. The Mesotaeniaceae, the simplest 

 of the unicellular Conjugatae, include 

 only a few genera. They are distinguished 

 from the following order by the cell wall 

 of the shortly cylindrical cells not being 

 formed of two halves. The mode of con- 

 jugation presents some differences. In 

 Cylindrocystis (Fig. 325) the protoplasts 

 of two cells fuse to form the zygote ; the 

 nuclei unite while the four chloroplasts 

 persist. Before germination the zygote 

 undergoes successive division into four 

 cells, which then escape. Reduction is 

 effected in the first nuclear division. In 



Spirotaenia the protoplasts of the conju- FlG . 3 -25.-^, Cylindrocystis BreUsonii ; the 

 gating cells first divide and the daughter 

 cells unite in pairs to form the zygotes. 

 Only two individuals arise from a zygote, 

 the other two being suppressed. 



2. The Desmidiaceae, which occur in 

 peaty pools, ponds, etc., 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 symmetrical 

 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.g. Cosuiarium, Fig. 326^4, ), sometimes stellate (Micrasterias, Fig. 326 

 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 

 pores. In some genera there is no constriction between the two halves of the cell. 

 This is the case, for instance, in the crescent-shaped Closterium moniliferum (Fig. 

 327 F), whose two chromatophores are elongated conical ribbed bodies, while in 

 each end of the cell there is a small vacuole containing minute crystals of gypsum 



nucleus is in the centre between two 

 large lobed chloroplasts with elongated 

 pyrenoids. J5, The zygote before, and C, 

 after the fusion of the nuclei. D, The 

 zygote before germination, with four 

 daughter cells. (After KACFFMANX.) 



