348 



BOTANY 



PART II 



6 



X 



\ 



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 Clostcriuiii moniliferum (Fig. 

 262, 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. ]\Iany Desmids are 



characterised by heliotactic movements ; 

 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. 

 Multijilication is effected by ceil 

 division. This is accomplished by the 

 formation of a partition wall across the 

 middle of the cell after the nuclear division 

 is completed. Each daugliter cell eventu- 

 ally attains the size and form of the mother 

 cell, by the outgrowth of a new half on 

 the side towards the new division wall 

 (Fig. 261, _^). _ 



In conjugation two cells approach eacli 

 other, and surround themselves with a 

 mucilaginous envelope. Their cell walls 

 rupture at the constriction, and parting 

 in half allow the protoplasts to escape ; 

 these then unite to form a zygospore. 

 The z3'gospores frequently present a very 

 characteristic aj^pearance, as their walls 

 are often beset with spines (Fig. 261, C). 

 The four empty cell halves may be seen 

 close to the spore. In some Desmidiaceae 

 the conjugating cells undergo a prelimin- 

 ary division, the daughter cells uniting 

 in pairs. 



The two sexual nuclei in the zygote do 

 not fuse until germination of the latter 

 is about to commence. The resulting 

 nucleus then undergoes division into four 

 nuclei, two large and two small. Only 



Fic 262. — Closterium. A, Zygote before yer- 

 miiiatioii showing the two nuclei not yet 

 united ; B, germinating zygote with the 

 nuclei united ; C, division into two cells t^o cells are formed from the zygote, eacli 



of which has thus two nuclei of different 

 sizes ; the smaller nuclei disappear. The 

 production of two cells on germination 

 thus appears to be derived from the divi- 

 sion into four cells seen in Mcsotaemum, 

 and to stand to the latter as a reduced form. 



3. Zygnemaceae. — In tliis family, all of which aie filamentous in character, 

 the gi'uus i^jdroijijra, witli its numerous species, is the best known. It is 

 commonly found in standing water, forming unattached masses of tangled green 

 filaments. Tlie fdaments exhibit no distinction of base and a])ex, and are 

 composed of simple rows of cells, which vary in length in different species. 



each containing one larger and one smaller 

 nucleus ; 7), further state of germination ; 

 K, young plants escaping from the cell- 

 mcnibrane ; F, Closterium mnnUifenim, ma- 

 ture ])lant. (.1-7'.' after Klebahn.) 



