222 MICROSCOPICAL MEMORANDA. 



the cells, may be frequently traced either in the same or different fila- 

 ments, which is alone sufficient to establish the reality of the existence 

 of this law of increase in this numerous section of the class Conferva. 

 The only Confervas to which I should for a moment hesitate to apply 

 this method of development, and I believe that it is applicable to them 

 likewise, are the branched species, to which such a means of increase is 

 less necessary, seeing that, unlike those with simple unbranched fila- 

 ments, they have innumerable terminal points of growth. Now I beg 

 to lay particular stress on this law of development, which is evidently 

 very important, inasmuch as it not merely goes to account for the rapid 

 growth of many species of Conferva ; for it is simultaneously in opera- 

 tion in each of the many hundred cells of which each filament of most 

 Conferva is composed ; but it likewise teaches us that much caution is 

 requisite in determining species, as it proves that the character most 

 relied on for this purpose, is one subject to very great variation ; that 

 is, the length of the joints. There is a limit, however, to this law of 

 development, which does not, in the section of the genus Conjugata, to 

 which reference has been made, allow of more than one or two divisions 

 of each cell, unless, indeed, the spiral tubes grow likewise in an equal 

 ratio, which may be the case, and then the division of the cells may be 

 frequently repeated. In those Conferva which do not contain spiral 

 tubes, the multiplication of the cells may go on to an almost endless 

 extent. To illustrate the importance of attention to this law of develop- 

 ment in determining species, I may observe, but for this timely discovery, 

 I should have described several species of Conjugata as distinct, which 

 are really not so, considering the length of the cells and number of 

 spiral tubes in the interior of each cell to be the most decided charac- 

 ters whereon to found specific differences. They are not so, however, 

 one of the most certain being the diameter of the filaments. But, 

 carying this law in view, it is not difficult to estimate the extent of 

 variations in length to which the cells are subject, first as ascertaining 

 what the primary length of the cell is. In the branched Conferva, there 

 are laws of development, some of them peculiar to each species, presid- 

 ing over the arrangement of the branches and cells, which have hitherto 

 escaped the scrutiny of man. Proc. Dublin Nat. Hist. Soc., June 1st, 

 1842, quoted in Ann. Nat. Hist., July, 1842, p. 431. 



Griffith's Observations on Santalum, Osyris, and Isoetis. In Santalum 

 the ovulum consists of a nucleus and an embryo-sac, prolonged both 

 beyond the apex and the base of the nucleus ; the albumen and embryo 

 are developed in the exserted part above the septum ; the mass of the 

 embryo is developed directly from the vesicle, which is the termination 

 of a pollen tube ; the seed (albumen) has no other proper covering 

 than the incorporated upper separable part of the embryo-sac. 



In Osyris the ovulum is reduced to a nucleus and an embryonary 

 sac, prolonged exactly in the same directions as in Santalum, but not 

 to such a degree anteriorly ; this anterior portion resembling exactly 

 the unchanged part of the sac of Santalum below the septum. The 

 albumen and embryo are formed outside the sac, and are absolutely 

 naked, or whatever covering they may have, did not enter into the 

 composition of the ovulum. 



