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conviction that it was the reproductive organ of the Staurastrum, only 

 after repeated observations and tracing the formation from the com- 

 mencement. 



" I then transmitted specimens to Mr. Jenner. He also at first 

 doubted the connexion between the fronds and the spinous body, but 

 after a minute inspection he confirms my observations in the follow- 

 ing words : ' I think I can say positively that I have traced them from 

 the first conjugation to the full-formed spinous body, and I now feel 

 satisfied that they are the spores or sporangia of the Staurastrum.' 



"The difficulty in detecting the connexion between the empty 

 fronds of the Staurastrum and the sporangium depends upon the te- 

 nuity of the connecting membrane ; the fronds also are generally at 

 a considerable distance, and soon become detached. 



"In the conjugated specimens the fronds are always smaller than 

 usual ; I have already noticed a similar fact respecting Tetmemorus 

 granulatus. 



" In Staui'astrum mucronatum the conjugated fronds are at first 

 closely connected by the formation of a bag-like receptacle which is 

 colourless and very thin, and therefore difficult of detection. As this 

 enlarges the fronds become more remote from each other, their seg- 

 ments partially separate at the constriction on the inner side, the en- 

 dochrome of both passes out, unites and forms an orbicular body 

 between them. In this state it resembles the sporangium formed in 

 some species of Closterium. At first it is inclosed in an orbicular 

 membrane larger than itself, but as it increases in size and density, 

 fine hairs make their appearance on the surface and gradually become 

 stout spines, the membrane lastly disappears and the sporangium ac- 

 quires its perfect state, covered with conspicuous acute spines. 



" In this stage the empty fronds of the Staurastrum seem scarcely 

 connected with the sporangium ; except that they are on opposite 

 sides of it, have an opening towards it, accompany it in its movements, 

 and always retain the same relative position. 



" The fi'onds of the Staurastrum in an end view had sometimes three 

 and sometimes four rays ; in conjugating a four-rayed variety would 

 often unite with one having three rays, and occasionally a frond might 

 be seen having four rays on one segment and three on the other. 

 These facts are another proof that the number of rays on a segment 

 cannot constitute a generic distinction in these plants. 



" It is probable that, under favourable circumstances, conjugation 

 takes place in all the Desmidieae. It is not uncommon in various 

 species of Closterium ; I have seen it in this genus and in Tetmemo- 



