Zygnemacece 117 



a single series of cylindrical cells forming an unbranched filament. 

 Rare instances of branching are known, but the branches have 

 been limited to short lateral outgrowths consisting only of a few 

 cells ; such outgrowths have been observed in the genera Zygnema 

 and Mougeotia 1 . Similarly, longitudinal septa of an incomplete 

 character have been observed in Zygnema pachydermum West, var. 

 confervoides West 2 . Rhizoid-like organs of attachment or haptera 

 are of frequent occurrence in young plants of Spirogyra and 

 Mougeotia, but have not been noticed in any of the other genera of 

 Zygnemaceas. They are usually simple or branched outgrowths 

 near the base of the filament, but in Spirogyra they may arise by 

 the modification of a conjugating- tube which has been protruded 

 by a cell some distance removed from those cells actually engaged 

 in conjugation. This is yet another proof of the increased activity 

 of the filament as a whole during conjugation. 



The ZygnemaceaB is divided into three sub-families, of which 

 the PyxisporeaB is only known from tropical Africa. The two 

 British sub-families are: 



Sub-family 1. Mesocarpece. Conjugation forming a zygospore which 

 immediately develops a sporocarp of several cells, one of which is the 

 spore (carpospore). The gametophyte is developed from this spore 

 after a period of rest. 



Sub-family 2. Zygnemece. Conjugation producing a zygospore which 

 after a period of rest develops directly into a new gametophyte. 



Sub-family I. MESOCAEPEJE. 



The plants of this sub-family are the narrowest and most 

 delicate of the filamentous ZygnemaceaB. There is a great varia- 

 bility in the diameter and relative length of the cells, and the 

 cell-wall is relatively thin. The cells contain a single chloroplast, 

 generally in the form of a thin axile plate, which may extend from 

 end to end or only occupy the median portion of the cell. Each 

 chloroplast contains several pyrenoids arranged in a single longi- 

 tudinal series. The chloroplasts of adjacent cells usually lie in the 

 same plane, so that a whole filament of cells may exhibit the full 

 breadth of the chloroplasts or it may be in a position such that 

 only the thin edge of the chloroplasts can be seen. The action of 

 light causes a rotation of the plate-like chloroplasts of Mougeotia. 



1 West & G. S. West in Ann. Bot. 1898, xlv, p. 32, t. iv, f. 17, 18, 19 and 41. 



2 West, ' Alg. from the W. Indies,' Journ. Linn. Soc. Bot. xxx, t. xiv, f. 5. 



