Mr. J. Ralfs on the British Desmidiese. 153 



a mucro. In a. and 7. the segments are transversely elliptic, but 

 in /8. they are lunate. The end view shows three mammillate 

 lobes or rays, each of which is terminated by a hair-like mucro. 



The smooth frond, the peculiar inflated or mammillate form of 

 the lobes in the end view, and the terminal hair-like points, well 

 characterize this species. 



P.S. — During the past summer I several times observed, scat- 

 tered amongst various Desmidiea, orbicular spinous bodies, the 

 colouring matter of which formed a dense green mass, which 

 tested by iodine assumed a dark blue colour, showing its vege- 

 table nature. On careful examination I found that each of these 

 bodies was usually accompanied by two empty fronds of Stauras- 

 trum mucronatum, and was placed between them, though not in 

 actual contact. Still it appeared so unlike any sporangium I was 

 acquainted with, that I arrived at the conviction that it was the 

 reproductive organ of the Staurastrum, only after repeated obser- 

 vations and tracing the formation from the commencement. 



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 

 following 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 

 tenuity of the connecting membrane ; the fronds also are gene- 

 rally 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 75?/- 

 memorus granulatus. 



In Staurastrum 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 segments partially separate at the constriction on the inner 

 side, the endochrome of both passes out, unites and forms an or- 

 bicular body between them. In this state it resembles the spo- 

 rangium formed in some species of Closterium. At first it is in- 

 closed in an orbicular membrane larger than itself, but as it in- 

 creases in size and density, fine hairs make their appearance on 

 the surface and gradually become stout spines, the membrane 

 lastly disappears and the sporangium acquires 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 



