ON THE DESMIDIACEJS. 75 



shaped themselves at the position of the old isthmus. These are, 

 at first, small, colourless, and destitute of all markings, Looking, as 

 Mr. Ralls says, like condensed gelatine. They, however, rapidly 

 increase in size and firmness, their contents assuming a grei n 

 colour, and their walls taking on the peculiar markings of the 

 species. At last, the parts thus formed having assumed the 

 shape and appearance of the original lobules, the two fronds, 

 which have been developed out of one, separate mostly before the 

 new semi-cells have acquired their full size. What part the nucleus 

 has in the process just described, I have never actually demon- 

 strated, but have little doubt but that it undergoes a division in 

 the very commencement, so that the new nucleus of each secondary 

 frond is formed out of one-half of the old one. 



In proportion as the form of the Desmid becomes simpler, so do 

 the peculiarities of its cell multiplication become less. In those 

 species which are simple cylindrical cells, there appears to be 

 nothing peculiar in the method of dividing, which, however, always 

 takes place through the centre of the cell, and subsequent growth 

 occurs, generally, only in the new found part. 



True sexual reproduction apparently does not take place as freely 

 in this family as the former process, for whilst I have seen hundreds 

 of cells undergoing the latter, it has not been my good fortune to 

 meet with conjugating specimens on more than two or three occasions. 



The process has, however, been studied very closely by De Bary, 

 Braun, Hofmeister, and others, and appears to consist generally in 

 a rupture of the outer wall of two cells, and the protrusion of de- 

 licate processes from an inner, often newly-formed coat, with sub- 

 sequent union of these, and consequently of the two cells, and 

 afterwards a condensation of the contents in the enlarged connect- 

 ing passage. The connecting passage between the fronds is really 

 a sporangium in which the spore is perfected, the contents of the 

 cell finally condensing it into a firm globe, and secreting around 

 themselves a thick coat. 



The after history of this spore has been very successfully studied 

 by M. Hofmeister, whose observations were made upon Cosmarium 

 tetraophthalmum l which he watched conjugating and forming a sort 

 of resting spore, which was perfected early in the month of July. 

 This was composed of a thick outer coat and green endochrome 

 lying within as a distinct ball, nowhere in contact with the investing 

 membranes. In three weeks time this chlorophyllous protoplasm 

 had divided into ellipsoidal masses, or primordial cells, which soon 

 surrounded themselves with cellulose walls, and became distinct free 

 cells in the granular fluid which filled the cavity of the original 

 spore. In August each of these masses was divided into two, and 

 in the month of September the process was repeated, so that out of 

 the original endochrome eight strong flattened primordial cells were 

 produced. Division in some specimens ceased here, and in others 

 took place once more, so that by the following spring all of the 



