Zygnemacece 121 



Indications of sexuality are to be found in the Mesocarpese, 

 but they are much less marked than in the Zygnemese. The 

 spores are often seen to be situated nearer to one gametangium 

 and the conjugating- tube of that gametangium to be thicker and 

 shorter than that of the other ; hence the former may be looked 

 upon as a female cell and the latter as a male cell. As these 

 indications of sexuality are scarcely discernible and often absent, 

 the Mesocarpese may be regarded as having lost almost all traces 

 of differentiation of sex. 



Genus Mougeotia Ag., 1824. [Staurospermum Klitz., 1843 ; 

 Mesocarpus Hass., 1845 ; Craterospermum Braun, 1855 ; Plagio- 

 spermum Cleve, 1868.] The thallus consists of cylindrical un- 

 branched filaments of elongated cells. The single chloroplast is 

 disposed_as an_axjle plate, extending from end to "ehcToTtne cell or 

 only occupying the median portion. The jjiyrenoids are numerous 

 and usually arranged in a single series. In ]ft. capucina the 

 chloroplast sometimes assumes the form of an irregular axile rod, 

 connected with the lining layer of protoplasm by fine colourless 

 strands, and the vacuoles contain a purple cell-sap. In some 

 species the carpospores are spherical, but in others they are quad- 

 rate and more or less flattened, with rounded or truncated angles. 

 Species of this genus were at one time referred to various genera, 

 such as Mesocarpus, Staurospermum, etc., according to the disposi- 

 tion of the sterile cells of the sporocarp and the form of the carpo- 

 spores, but all the supposed generic differences have been found 

 by Wittrock to be present in one and the same filament of 

 Mougeotia calcarea Wittr. Throughout the entire genus there is 

 great variability in the relative size of the carpospore and the 

 sterile cells of the sporocarp. 



In mountain tarns and lakes species of this genus are extremely 

 abundant, and they flourish in the summer months in small pools 

 on the mountains up to 3,000 ft. elevation. In these situations 

 the plants rarely conjugate and they are kept alive through the 

 winter largely by the formation of resting-cells or ' cysts,' which 

 are of the same form as the ordinary vegetative cells. In the 

 plankton of large lakes the filaments are often much twisted and 

 coiled. 



There are about 15 British species of the genus, of which M. scalaris Hass. 

 (diameter of fil. 32 35 /*) is the largest and M. elegantula Wittr. (diameter of 

 fil. 3'5 4'5 fi) is one of the smallest. The two most abundant species are 



