290 Ulotricltales 



Reproduction occurs by zoogonidia developed singly (or more rarely in 

 twos, threes or fours) in the vegetative cells (fig. 185 E G). They are 

 ovoid or subellipsoid, biciliated, and possess an irregularly lobed chloroplast 

 the limitations of which are usually indefinite. As a rule most of the cells 

 of a filament produce zoogonidia simultaneously, their formation and ultimate 

 escape only occupying about an hour. The escape of the zoogonidia in the 

 different species brings out clearly the differences in the structure of the 

 cell-wall. In M. tumidula the entire filament becomes disarticulated into 

 H-pieces (fig. 185 E} and the zoogonidia swim quickly away; in M. stagnorum 

 the lateral walls of the cells gradually swell up and become mucilaginous, 

 the mucus soon becoming diffluent and allowing the zoogonidia to escape 

 (fig. 185 G); in M. floccosa there is an intermediate condition in which the 

 filament becomes disarticulated in rather an irregular manner, some portion 

 of the lateral walls being converted into mucilage (fig. 185 F). After 

 swarming for a brief time the zoogonidia come to rest and at once germinate. 

 The young filament may be attached, but usually it is not. In any case 

 the young plant shows a distinct differentiation into base and apex 

 (fig. 185 H J). 



Thuret ('50), in his original account of M. floccosa, figures 8 or 16 minute 'zoogonidia' 

 arising from each cell of the filament. Such is not the case in any species of Microspora 

 examined by the present author not even in M. floccosa. In all observed cases, with the 

 exception of M. amaena, only one biciliated zoogonidium was formed in each cell. From 

 a careful comparison of Thuret's figures 5, 6 and 7 (on PI. 17 of his work) with the 

 formation of zoogonidia as observed by others in M. floccosa, it would not appear un- 

 reasonable to suggest that the minute elongated ' zoogonidia ' figured by him were really 

 isogametes and that his fig. 7 represents the germination of the zygote. 



Spherical or slightly compressed aplanospores with strong cell-walls are 

 sometimes formed singly within the vegetative cells (fig. 184 C and F). 

 Akinetes are formed abundantly in some species, such as M. floccosa. They 

 are subquadrate or rectangular, slightly tumid, and provided with thick 

 walls (fig. 91 D). Both aplanospores. and akinetes generally occur in long 

 chains, and both are resting-spores. 



Gametes have not been observed, unless the above suggestion concerning 

 Thuret's ' zoogonidia ' of M . floccosa, is correct. 



The systematic position of the genus Microspora has been a controversial question for 

 some time. Bohlin ('01) regarded it as the type of a separate order, the Microsporales, 

 and in this he was followed by West ('04). Blackmail & Tansley ('02) placed the genus in 

 the family Microsporacese of the Ulotrichales, and both Oltmanns ('04) and Wille ('09 B) 

 referred it directly to the Ulotrichaceee. 



A careful investigation of the British species upholds the position assigned to this 



genus by Blackmail & Tansley. It cannot with justice be placed in the Ulotrichacese, and 



yet in view of the occurrence of a reticulated chloroplast in the axial cells of Draparnaldia 



platyzonata (consult fig. 190 B) and the usual absence of pyrenoids from certain species of 



Ulothrix (such as U. svMilis and U. szqualis), it is probably best to regard the genus as <* 



