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liad found on the walls of a greenhouse at Kew. In sizo the species varied 

 from that of a pin's head to a diameter of two or threo inches ; but all of 

 them had a similar structure, all consisting of a mass resembling isinglass 

 jelly, enclosing a number of wavy threads, composed of small globose cells, 

 attached end to end like a chain. The normal form of these cells was 

 globose ; if they were at any time found to be square or cubical, the varia- 

 tion was the result of some sort of compression to which they had been sub- 

 jected. No other organs had been observed, so that the nostoc might be 

 said to consist of a mass of jelly enclosing a number of moniliform threads. 

 If these chains were examined by the microscope, it would be found that at 

 variable distances larger cells were interjected, and these were supposed at 

 one time to be spermatia. When seen under favourable circumstances it 

 was possible to trace motionless cilia extending from these larger cells — 

 now known as heterocysts ; but notwithstanding the attention which had 

 been given to them, their function was still as great a mystery as ever. 

 They were in most cases easily recognised by their size, but they could also 

 be distinguished by their colour, having a yellow endochrome instead of a 

 green one like the other cells. When mounted, however, all colour dis- 

 appeared, so that if the cells and the heterocysts happened to be of the same 

 size it was then difficult to distinguish them ; but supposing that there was 

 no visible sheath enclosing the thread, a solution of iodine would be of use, 

 as it would react more on one than on the other. Though their purpose was 

 so little known, they were of considerable service in determining species. 

 During the growth of the nostoc some remarkable changes took place in 

 these cells, two or three or more of them in a thread would sometimes be 

 seen to enlarge themselves, and having done this they would divide and 

 break off, and at last be converted into spores. Why they should thus 

 differentiate themselves was at present a puzzle, but it was clear that they 

 did so, and that they became filled with granular matter, and finally 

 germinated. Another mode of reproduction had been observed— portions of 

 the threads between two heterocysts would break away, constituting what 

 is termed a hormogone, and leaving the heterocysts in the jelly, would 

 emerge and become living and moving, and might be seen to wriggle about ; 

 then they became broader and broader until they constituted a double 

 thread of cells through their entire length, and then separated down the 

 middle, becoming two threads ; they then gathered a jelly about themselves, 

 and this was the initial stage of a new nostoc. It would be seen, there- 

 fore, that they had a more peculiar mode of reproduction, exhibited by this 

 genus, than was found amongst others of the algae. 



Mr. Michael said there was one little point which he did not thoroughly 

 catch, as to the lateral widening in one direction giving rise to a new set of 

 cells. Was this a widening of the original cell or a dividing of a cell ? 



Dr. Cooke said the process differed entirely from the ordinary method, 

 which would be by the dividing of a cell ; but in this case they had the cell 

 itself gradually widening, instead of constricting, and this went on until it 

 had become as large as two cells before any constriction took place. No 

 nuclei had as yet been found. 



