26 Prof. H. Karsten an the Formation, 



they are not quite constant in adjoining joints of the selfsame 

 individual plant when in a normal state of nutrition ; nay, even 

 in the same joint-cell of Spirogyra quinina, one half is occupied 

 by one and the other by two bands of chlorophyll. 



The structure of these chlorophyll-bands, however, vanes, 

 as well as their disposition on the cell-wall. It is also de- 

 pendent on the nature of the nutritive material and on the 

 phase of development of the joint-cell, in the same way as the 

 other organized contents of its interior. 



The contents of the joint-cells of Spirogyra are commonly 

 described as a fluid matter surrounded by spirally twisted 

 bands of chlorophyll, to which, at the centre of the cell, a 

 nucleus is suspended by means of mucous threads. 



This interpretation of the structure of Spyrogyra labours 

 under the same defects as the one heretofore entertained with 

 respect to Cladophora, as a few experiments will prove. 



In the joint-cells of Spirogyra we find, even with more 

 distinctness than in those of Cladophora, secretion-cells of dif- 

 ferent sorts, some filled with colourless fluid occupying the 

 central space of the cell, and others containing a greenish 

 mucus deposited on the surface in the form of the so-called 

 chlorophyll-bands. 



The spiral bands, which are usually channelled, sometimes 

 furnished with a median rib or keel and often with a dentate 

 margin, are produced, according to Kiitzing (Phycologia Gene- 

 ralis, 1843, p. 275), by the laceration of a gonimic substance at 

 first deposited on the tender growing cell in a homogeneous 

 manner, the laceration being due to a rapid extension and growth 

 of the cell. 



Mohl (Vermischte Schriften, 1845) likewise attributes the 

 spiral hands of Spirogyra to the division of formless chloro- 

 phyll composed of a delicate green jelly-like substance. 



My first investigation (Wiegmann's Archiv, 1843) of the 

 production of these chlorophyll-bands in the elongated extre- 

 mities of the cells of Spirogyra led me to believe that they 

 originated from cells the membranes of which became condensed 

 around a mucoid yellowish mass, enclosing a nuclear vesicle, 

 and that the cells so formed proceeded to elongate, whilst their 

 mucoid contents acquired a green colour and arranged them- 

 selves with the existing spiral bands. The former part of this 

 hypothesis is erroneous, partaking as it does of the erroneous 

 views respecting cell-formation then prevaiUng. A year afterwards 

 I pointed this out, and maintained then, as now, that the mem- 

 brane did not form around the mucus, but was present from the 

 first, investing the colourless and rather turbid mucus, which, as 

 the cell-wall increased in thickness, acquired first a yellow and 



