Zygnemacece 123 



spore is formed by the rejuvenescence of the contents of the 

 mother-cell, and there is a greater difference in size between the 

 spores of species of this genus than can be accounted for by the 

 difference in cubical capacity of the mother-cells. 



Plants of this genus are of much rarer occurrence than those of Mougeotia ; 

 in fact, they are amongst the rarest of the Conjugatse. G. notabile (Hass.) 

 Wittr., with vegetative cells 12 15 p. in diameter, has not been seen since its 

 discovery by Hassal in 1845. G. ventricosum Wittr. (fig. 45 G J), with veg. 

 cells 7'5 8'6 p in diameter, is known from Ireland; and G. Boodlei W. & 

 G. S. West (fig. 45 A F), with veg. cells 5 5 '5 /* in diameter, has been found 

 in Surrey. The aplanospores of G. ventricosum are likewise considerably 

 different from those of G~ Boodlei and they are sometimes produced by the 

 swelling of the free end of the terminal cell of a filament. 



Sub-family II. ZYGNEME^E. 



The plants of this sub-family consist of unbranched filaments 

 of cells similar to those of the Mesocarpese, but usually of much 

 larger size. There is a lining layer of protoplasm in each cell and 

 the nucleus is situated in the central portion of the cell, embedded 

 in a mass of protoplasm which is connected with the lining layer 

 by numerous radiating strands. Gerassimoff 1 finds certain cells of 

 Spirogyra majuscula to contain two ordinary nuclei or one com- 

 pound nucleus. The chloroplasts, which contain prominent pyre- 

 noids, are somewhat variable in form and disposition ; in Debarya 

 there is a single axile plate similar to that of Mougeotia', in 

 Zygnema there are two star-shaped chloroplasts suspended in the 

 middle line of the cell; and in Spirogyra the chlorophyll is 

 arranged in one or more bands twisted spirally round the interior 

 of the cell- wall. The chloroplasts of Spirogyra are very variable 

 and special reference is made to this feature under the genus. 



Vegetative multiplication takes place as in the Mesocarpese by 

 the breaking up of a filament into its constituent cells, or into 

 groups of cells, which then grow into new filaments. 



Asexual reproduction sometimes takes place by the formation 

 of aplanospores, which are produced from the contents of a single 

 cell. They are more frequently found in Zygnema than in Spiro- 

 gyra, and until the discovery of Z, spontaneum Nordst. in West 

 Africa 2 , the only known method of reproduction of this species was 



1 Gerassimoff in Bull. Soc. Imp. Nat. Moscou, 1897. 

 3 W. & G. S. West in Journ. Bat. Febr. 1897, p. 15. 



