Desniidiaeese 367 



noticed in a single semieell of Pleurotxnium nodulosum var. coronatum (G. S. W., '99), 

 each one being partially filled with an incessantly moving mass of minute granules, which 

 move freely in the vacuole, but always collect towards its base. These granules are of a 

 pale yellow colour and appear brown in a thin stratum ; but when present in immense 

 numbers they give the Desmid almost a black appearance. The development in quantity 

 of these moving granules is a pathological condition and is associated with the gradual 

 disintegration of the chloroplasts. At the same time the cell-sap in the large vacuoles 

 very often becomes coloured violet with phycoporphyrin, a pigment which occurs normally 

 in the cell-sap of very few Desmids. This pathological state can be seen frequently in 

 Penium, Cosmarium, Euastrum, Micrasterias, Staurastrum and other genera. 



The normal method of increase in the Desmidiaceae is a vegetative multi- 

 plication by simple cell-division, which occupies about a day^ in the smaller 

 and simpler types, but several days in the larger species with more complex 

 outlines. The essential points of the cell-division of Closterium were well 

 described by Fischer ('83) and more recently a very detailed account of the 

 process as it occurs in Closterium Ehrenbergii and Cl. moniliferum has been 

 given by Lutman ('11). The details of cell-division are not quite the same in 

 all Desmids and it is a curious fact that so far they have only been completely 

 worked out in the unconstricted forms, whereas fully 90 per cent, of known 

 Desmids are more or less deeply constricted. 



In the more usual type of Desmid with a median constriction the first 

 step in cell-division is a division of the nucleus and, as this approaches com- 

 pletion, there is an elongation of the isthmus causing a slight separation of 

 the semicells. The elongated isthmus generally becomes slightly swollen and 

 across its median part a new cell-wall is laid down 1 . The parts of the isthmus 

 on either side of this new wall are destined to grow into the new semicells. 

 They rapidly become more turgid and soon grow into adult semicells. The 

 lobulation or ornamentation of the species gradually makes its appearance on 

 the new semicells as they assume the normal size. As a rule the two new 

 semicells, and therefore the daughter-cells resulting from division, remain 

 attached until they are full-grown. Chloroplasts appear in the new semicells 

 at a comparatively early age, but at present there is no exact information 

 concerning their origin. The pyrenoids in many cases certainly arise de 

 novo. 



In the unconstricted genus Closterium 2 , the only genus of Desmids in 

 which cell-division has been minutely studied, the process of division, although 

 similar in essential points, differs in details from that which occurs in con- 

 stricted Desmids. Lutman ('11) has shown that in Closterium Ehrenbergii 

 and CL moniliferum the first visible appearance of division in any individual 



1 The details of the early stages of cell-division yet require careful investigation. 



2 There are several slightly constricted species of this genus; notably Closterium Braunii 

 Reinsch ('67) and Cl. subcompactum W. & G. S. West ('02). 



