CRYPTOGAMS 



:J4f. 



zygotes; in Mcsotacii-inm. and ' 'i/lindrocystis the protoplasts conjugate without a 

 preliminary cell-division. .Similar differences are shown by the Diatomeae. The 

 zygote on germination divides into four new individuals. 



'1. 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 

 symmetrical halves, separated, 

 as a rule, from each other by 

 a constriction. Each half 

 contains a large, radiate 

 chromatophore, or a number 

 of plate - like chromatophores 

 united into one. 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. COK- 

 marium, Fig. 271 A, B) some- 

 times stellate (Micrasterias, 

 Fig. 271 ]'>). The cell walls, 

 which, as in the Diatoms, con- 

 sist of separate halves, are 

 frequently beset with wart- or 

 horn-like protuberances and 

 often provided with pores. In 

 some genera there is no con- 

 striction between the two 

 halves of the cell. This is the 

 case, for instance, in the cres- 

 cent-shaped Closterium monili- 

 /'/ /?i( Fig. 272 F), whose two 

 chromatophores consist of six 

 elongated plates, united in 

 the long axis of the plant, 

 while in each end of the cell 

 there is a small vacuole con- 

 taining minute crystals of 

 gypsum in constant motion. 

 Many Desmids are character- 

 ised by heliotactic movements ; they protrude fine mucilaginous threads through 

 the cell walls, by means of which they can push themselves along, and take up a 

 position in a line with the direction of the incident rays of light. 



Multiplication is effected by cell-division. This is accomplished by the forma- 

 tion of a partition wall across the middle of the cell after the nuclear division is 

 completed. Each daughter cell eventually attains the size and form of the mother 

 n 11. by the outgrowth of a new half on the side towards the new division wall 

 (Fig. 271 A}. 



J) 



Kiii. _'. I. .1. I'IIXHIII fin HI i-oelatuni , dividing. B, C, 

 liotri/tis. Cj, two cells at right angles prepariii 

 jugation the lower cell shows the conjugation canal 

 gametes fused into the young zygote ; Cy, mature />; 

 //, .U ii'i'usii riii-- t'l-ii, iiirliteiisix. (After RAI.KS : C] 

 alter DK JURY.) 



for con- 

 ('.,, 



