REJUVENESCENCE IN NATURE. 295 



annular slit, the protruded internal membrane entering 

 into the conjugation becomes very strongly developed. 

 The spores become free by the disappearance of the 

 enlarged saccate conjugation-cell; this also sets the 

 halves of the old cell-membrane free, from which the 

 internal membrane does not seem to be drawn out as in 

 Tetmemorus. This case occurs in Euastrum* and the 

 genera intimately allied to, and difficultly separable from 

 it, Micr aster las, \ Cosmarium,\ Xanthidium,^ Staurastrum 

 (Phycastrum, K.) || The complete separation of the 

 halves of the mother-cell membrane does not, however, 

 seem to be constant, for Ralfs gives figures of the same 

 genera, and even of the same species, as those above 

 cited, which exhibit halves connected together at one 

 side.^f 



j8. The halves of the dehiscent cells separate only at 

 one side, in order to emit the connecting canal formed 

 out of the internal membrane, not expanded into a sac ; 

 they do not become detached. The spores are formed 

 either in the middle of the connecting canal (as in Hya- 

 lotkeca** Bambmina,\\ and Isthmosira (Sphcerozosma, 

 Ralfs), j j or in one of the halves, in a manner similar to 

 that we have seen in Zygnema and most of the Spirogyra, 

 a case which is only known with certainty in Didymo- 

 j3riuM among the Desmidiaceae, but perhaps occurs also 

 in the genus Desmidium. || || 



* Ralfs, 1. c., t. 12, (Euastr. oblongum;) t. 14, f. 5, (E. pectinatum^ &c. 



f Ibid., t. 6, f. 1, (Micrasterias denticulate/, with the mace-like spores 

 beset with forked spines ; the conjugation itself is not represented.) 



J Ibid., t. 16, f. i, (Cosmar. Botrytis;} f. 2. (C. margaritiferum) &c.; 

 Nageli has also described the conjugation in a species of this genus, his 

 C.vupestre, (1. c., p. 118, t. vii, f. 6.) 



Ibid., t. 18, (Xanth. armatum.) 



|| Ibid., t. 20, f. 5, (St. dejectum;} t. 21, f. 5, (St. orbiculare ;~) t. 22, f. 3, 

 (St. hirsutum-) f. 9, (St. polymorphum ;) t. 23, f. 9, (St. brachiatum,} &c. 



^[ E. ff., of Cosmarium maryaritiferum, t. 33, f. 6 ; Co&m. Broomei, t. 33, 

 f. 7 ; Arthrodesmus Incus, t. 20, f. 4. 

 * Ralfs, 1. c., p. 53, t. 1. 



ff Ibid., p. 59, t. 3, (as Didymoprium Borreri.} 



JJ Ibid., p. 67, t. 6, f. 2 (/. excavata;} t. 32, f. 2, (/. vertebrata.) 



| Ibid., p. 55, t. 2, (Didym. Grevillii.) 



|| || Ibid., t. 4, where Ralfs represents spore-like globules in the interior 



