394 



BOTANY 



in constant motion ( 25 ). Many Desmids are characterised by heliotactic move- 

 ments ; they protrude fine mucilaginous threads through the cell walls at their 

 ends ; by means of these 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- 



FIG. '326. A, Cosmarivm coelatum, dividing. B, C, 

 Cosmarium botrytis. C\, Two cells at right angles 

 preparing for conjugation the lower cell shows 

 the conjugation canal ; C%, gametes fused into the 

 young zygote ; C$, mature zygote ; D , Micrasterias 

 crux melitensis. (After RALFS ; C%, C% after DE 

 BARY.) 



FIG. 327. Closterium. A, Zygote before 

 germinatio.ii showing the two nuclei 

 not yet united ; B, germinating 

 zygote with the nuclei united ; C, 

 division into two cells each contain- 

 ing one larger and one smaller 

 nucleus ; Z>, further state of ger- 

 mination ; E, young plants escaping 

 from the cell-membrane ; F, Clos- 

 terium moniliferum, mature plant. 

 (A-E after KLEBAHN.) 



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 

 cell, by the outgrowth of a new half on the side towards the plane of division 

 (Fig. 326 A). 



In conjugation two cells approach each other, and surround themselves with a 



