ALGAE PROTOCOCCACEAE. 



39 



If, in conclusion, we compare the mode of proceeding in the case of Pandorina with 

 that in Endorina and Volvox, it appears that the differences are connected chiefly 

 with the form of the fertilising organs. Of the small coenobia in Pandorina, the cells 

 of which become gametes, some are male, others female ; therefore it is most probable 

 that the gametes from one and the same coenobium do not conjugate together ; in like 

 manner the bundles of spermatozoids in Eudorina ^ and Volvox are simply small 

 male coenobia and are strikingly distinct from the female. In Volvox there is the 

 further complication that most of the cells in a coenobium remain sterile and do not 

 become sexual. The term ' coenobium ' is applied in one sense in the case of the 

 Volvocineae and in another in that of the Hydrodictyeae (see the next section). It 

 has been shown that the coenobia of Volvox are not formed by combination of originally 

 separate cells, but by division of a single cell. The term admits of justification, if we 

 bring into the comparison such Volvocineae as Chlamydomonas, which live as single 

 isolated cells and agree entirely in structure with a cell of a coenobium of Volvox ; the 

 expression therefore is properly only comparative. 



3. PROTOCOCCACEAE. 



Under the name Protococcaceae we include a number of unicellular fresh-water 

 Algae, which fall naturally into two sections, the Hydrodictyeae, in which several 

 cells, originally separate, unite to form cell-families (coenobia or colonies), but have 

 not, like the Volvoc ineae, the power of movement, and the Eremobiae, in which the 

 isolated unicellular individuals live apart from one another often in the cavities 

 of other plants, living in but not on their host (' Raumparasitismus.') 



FIG. 19. Pediastrum granulatum (magn. 400 times). A a disk of cell adhering to 

 one another ; at g the innermost layer of the wall of a cell is just issuing from the cell, 

 and contains the daughter-cells formed by division of the green protoplasm ; at t various 

 states of division of the cells ; sp slits in the walls of cells which have discharged their 

 contents. B the inner lamella of the wall of the mother-cell disengaged from the cell 

 and much enlarged, b contains the daughter-cells g, which are in lively swarming motion. 

 C the same family of cells four hours and a half after its birth, four hours after the small 

 cells have come to rest; they have arranged themselves into a disk, which is already 

 beginning to develope into such a one as A. After A. Braun. 



a. HYDRODICTYEAE 1 . The genera Pediastrum and Hydrodictyon may be 

 taken as representatives of this group. They have a sexual mode of reproduction, 

 actually observed as yet only in Hydrodictyon, and an asexual. Pediastrum consists of 



1 On Pediastrum, &c. see Braun, Die Verjungung in der Natur 1857, and, Algarum unicellularium 

 genera nova vel minus cognita. Leipzig 1855. Pringsheim, Ueber die Dauerschwarmer des Wasser- 

 netzes (Monatsbericht der Berlin. Akad. Dec. 1860). The conjugation of the gametes of Hydro- 

 dictyon was observed by Suppanetz. 



