ARCHER AND DIXON, ON DESMIDIACEJE. 91 



Herbert Thomas (loc. cit., plate v, figs. 13 and 21) in the 

 object of her study, Cosmarium margaritiferum and C. 

 Thwaitesii. I have seen similar in other species, especially 

 smooth ones, such as Cosmarium Ralfsii, Cosmariumundulatinn, 

 &c. It does not appear to me evident what this pellicle-like 

 production may be. It seems to me to be possibly only, as it 

 were, the matrix of gelatine formed during the act of division 

 and fresh growth, which may have become denser and firmer, 

 and from which the fronds then emerging and leaving it 

 behind, gives rise to a membranous appearance with the 

 doubly cup-shaped outline ; or it may possibly be a secretion 

 deposited superficially during growth, even more comparable 

 than the usual gelatinous investment to what in the higher 

 plants is called " cuticle," and which in them is occasionally 

 separable by peculiar treatment. In the growing Desmidians 

 referred to, however, this investing pellicle-like production 

 does not extend beyond the new segments, ceasing at the 

 sutures. It is sometimes particularly remarkable in Docidium 

 (in D. clavatum I have noticed it), because it forms two 

 lengthened tubes (each, indeed, as long as one of the new 

 segments, and closely applied thereto), in apposition at the 

 closed ends and open at the opposite, from out of which the 

 fronds are to emerge, and indistinguishable till the process has 

 commenced. This may be witnessed under the microscope 

 during its accomplishment, when it is seen that near the open 

 ends the cast-off tube is of a slightly undulated outline; being, 

 in fact, a cast from the new segments. In this species the 

 segments appear to me to possess one or two slight basal 

 inflations, though described as with only one in 'British 

 Desmidieae.' I believe it, however, to be quite distinct from 

 D. Ehrenbergh. Whether a hyaline pellicle-like investment, 

 sometimes met with, entirely surrounding certain single indi- 

 viduals, and apparently like a loose tunic, is anything analo- 

 gous to the above-mentioned production, I cannot pretend to 

 say ; but I have occasionally seen such in some species, — for 

 example, Euastrum didelta. This involving cyst does not 

 follow the boundary of the form, but is of a rounded or oval 

 outline, and generally, when I have noticed it, the contained 

 frond seemed to have lost vitality, the endochrome being 

 brownish. What I allude to is not to be mistaken for the 

 gelatinous investment surrounding the frond in so many Des- 

 midian species, being altogether different from anything of 

 the sort seen in fresh specimens. Possibly, like the former, 

 it may be due to its consolidation — here producing, by a pro- 

 cess of superficial condensation, as it were, a kind of skin, 

 from beneath which the intervening gelatinous substance has 



