426 Mr Gustav Mann's 



preceding night, the threads exhibited during the day a helio- 

 tropic tendency, they approached the side of the vessel next 

 the window only to grow away from that side during the 

 night. I believe the nutation to be brought about in the 

 following way : — As soon as the strong stimulus " light " has 

 stopped acting on the cells, each cell, and thus the whole 

 thread, will tend to procure for itself as large an area of 

 water as possible, for the larger the area of water the more 

 readily can respiration be carried out, and the more oxygen 

 there is taken up by each cell the more energetic will be the 

 metabolism of starch into directly available material. 



This material will be consumed, firstly, by those elements 

 of the cell which are essential for the maintenance of the 

 life of the cell as an individual ; any surplus of nourishment 

 will be used for purposes of cell division or multiplication ; 

 or, to put it more clearly, it is of the highest importance to 

 recognise that there are in each cell definite structures 

 essential for division, i.e., a reproduction of the cell, namely, 

 the nucleolus with its contents ; and other structures, which 

 bring about this division by procuring and elaborating nutri- 

 tive substances, namely, the nucleus proper, the cytoplasma, 

 and the cell wall. Thus to distinguish in each cell between 

 a reproducing and a vegetating element. The view just 

 stated allows us to understand why, during the colder months 

 of the year, cells of Spirogyra should attain a larger size 

 than during the warmer months ; for, during winter, each 

 cell will have to combat low temperature, want of light, 

 and similar conditions unfavourable for procuring and ela- 

 borating nutritive material, and hence the vegetating element 

 of the cell will be specially developed to save at least the 

 life of the individual cell. 



It is only natural that under such conditions a cell should 

 sometimes have enough energy to start division, without, 

 however, being able to complete it, as is evident in those 

 cases, not uncommon during winter, in which two nuclei 

 are found in one large cell with either no indication of 

 a new cell wall, or with only the rudiments of a cell 

 plate attached to the inner aspect of the old wall (fig. 3, 

 Plate II.). That in these cases we deal with cells which 

 would not have formed a complete partition, had they not 

 been disturbed in doing so by fixing the threads for micro- 



