68 THE JOURNAL OF BOTANY 



SO that just as apical vacuoles are not constantly present in Clos- 

 terium, they are not constantly absent in Hoya. In Senium, 

 likewise, apical vacuoles are present in some but absent in other 

 species. The absence of granules in the terminal vacuoles of 

 H. anglica is hardly to be considered of much importance ; under 

 somewhat different external conditions it is possible that they would 

 develop. In Closterium, in one and the same species, their presence 

 and number is often variable ; Liitkemiiller, for example, figures (/. c. 

 t. i. fig. 11) an individual of CI. pusillum var. moiiolifhum with no 

 granule in the end-vacuoles, although normally a single moving 

 granule in each vacuole is present in this Desmid. 



As already remarked, a very large number of cells were in con- 

 jugation. Two cells about to conjugate approximate and become 

 embedded in a mass of mucilage. The mucilaginous masses observed 

 floating in the water consisted mainly of agglomerated conjugating 

 cells and zygospores, while vegetative cells were mostly free in the 

 water, and appeared to develop very little if any mucilage. Each of 

 the two conjugating cells puts out a protuberance, which, by local 

 dissolution of the cell-wall, becomes a pore, through which the cell- 

 contents emerge as a gamete (E). The pore is circular, and the 

 cell-wall round it has a slight outward curl, making the aperture 

 somewhat tubular; this apparently may be produced at any point in 

 the cell-wall, except at the thickened extremities. The zygospore (F) 

 is globose, with a thick, hyaline, smooth wall. This agrees with 

 JR. ohtusa { — CI. ohtusum Breb.), observed in conjugation by Kirch- 

 ner*, who states that the zygospores of this species are *' kugelig, 

 glatt, in eine dicke Schleimlmlle eingesehlossen." G. T. Harris, 

 however, who found B. ohtusa var. montana W. & G. S. West in 

 conjugation in bogs on Dartmoor, records the zygospore as ellipsoid 

 and smooth (size 22 ^ X 15 /x) t ; but the figure given by him (t. xix. 

 fig. 11) does not show the pores in the cell- walls of the empty cells, 

 through which presumably the gametes emerged. As far as I am 

 aware, these are the only records of species of -Boy shaving been found 

 in conjugation. 



In its mode of conjugation R. anglica recalls Oonatozygon rather 

 than Closterium, but the characters of cell-wall and chloroplast at 

 onco distinguish it from the former genus. In the fact that it has 

 apical vacuoles, concave ends to the chloroplast, and the latter, in 

 mature cells, frequently completely divided in the middle, R. anglica 

 resembles Closterium and Fenium much more closely than do any 

 of the other species of Boga. Indeed, it seems that the chief distin- 

 guishing characters on which i?oy« can be retained are (1) the simple 

 structureless nature of the cell-Mall, and (2) the fact that division of 

 the chloroplast into halves is delayed until the cell has reached mature 

 age, or even mitil it is about to divide. 



♦ Beitr. Algenflora Wiirt. in Jahresh. Ver. f. nat. Naturk. Wiirttemberg, 1880, 

 p. 173. 



t '• The Pesmid Flora of Dartmoor." Joiirn. Quekett Micros. Club, xiii. 254 

 (1917). 



