OF THE DESMIDIE^. 19 



these daughter-cells of the last generation from the form of cii^cular lenticular 

 bodies into two-lobed ones like the mother-plant. Here the young Cosmaria, 

 whose diameter amoimted to scarcely ith or ith of that of the mother-plant, 

 were set free by the very gradual solution of the membrane of the spore. A 

 similar process very probably occurred in Cosman'um tetraoplithcdmum, but 

 could not be observed there, from the circumstance that aU the materials had 

 been used up in the investigation. 



'' These facts place it beyond doubt that the contents of the spores produced 

 by the conjugation of two individuals of Cosmarium, are transformed by 

 repeated binary division into eight or sixteen daughter-ceUs, which assume the 

 form of the mother- cell, and finally become free by the solution of the wall 

 of the spore. Such behaviour of the spores had indeed been rendered pro- 

 bable before, by the discovery of the vesicular structure observed by Focke 

 and Ealfs, which enclosed a nimiber of small Chsteria, for the most part 

 beginning to divide. But the certainty which can only be given by direct 

 observation of the development was altogether wanting. 



'' The development of four daughter-cells in the interior of spores produced 

 by the conjugation of tvv^o individuals (with participation of the whole of the 

 cell-membrane), has been demonstrated by Alex. iJraun for the Palmellacean 

 Pahnoglcea macrococca, Kiitz. (?)." 



Sporangia are the only portions of Desmidieae of past eras which have been 

 preserved to us in a truly fossil condition. Ehrenberg discovered certain 

 orbicular and spinous bodies in flint, some of which he referred to the genus 

 Xanihidium among the Desmidiese, and others to Pyxidicula among the Dia- 

 tomese. However, as Mr. Ealfs remarks (p. 13), this association is, no doubt, 

 erroneous, since in tnie Xanthidia the cell is compressed, bipartite and bi- 

 valved, whilst in these fossils it is globose and entire, and there can be no 

 doubt that they are fossil sporangia (XYII. 506 to 515). 



To quote ]\Ii\ Ralfs's account (p. 13) — " The fossil forms vary like recent 

 sporangia, in being smooth, bristly, or furnished with spines, which in some 

 are simple, and in others branched at the extremity. Sometimes, too, a 

 membrane may be traced even more distinctly than in recent specimens, either 

 covering the spines or entangled with them. Some writers describe the fossil 

 fonns as having been silicious in their living state ; but Mr. Williamson in- 

 forms me that he possesses specimens which exhibit bent sj^ines and torn 

 margins, and thus whoUy contradict the idea that they were silicious before 

 they were imbedded in the flint." 



Another mode of propagation is presumed to take place by means of the 

 active molecules seen within the fronds of Desmidieae — in other words, by 

 zoospores, as happens in many families of Algae. M. Morren advanced this 

 notion, and imagined the minute particles which he denominated " propa- 

 gules," to be at once transformed into small fronds. Mr. RaKs countenances 

 the opinion so far as to say that the escape of the granular contents of the 

 mature frond is probably one mode of reproduction. He, however, likewise 

 regards (as Prof. W. Smith observes) the swarming of the granules as 

 identical with the movement of the zoospores, and confesses to his ignorance 

 of the history of the motile granules after their escape. But we perfectly 

 coincide with Prof. Smith that the swarming of the granules within a mature 

 frond is in most cases " a disturbance attendant upon the decay of the 

 granular mass," and not a phenomenon connected with reproduction. Still 

 our acquaintance with the swarming granules, particularly after their escape 

 from the frond, is so imperfect that it is useless to speculate on their func- 

 tional purpose. 



Ehrenberg, to carry out his hypothesis of the animal nature of Desmidieae, 



c2 



