260 University of California Publications. [Botany 



they are to be found in every plant. Since the other characters 

 are constant in all collections, these bodies probably do not fur- 

 nish a specific character, but only a physiological one. These are 

 probably the refractive bodies spoken of by Gomont in his de- 

 scription of the species. Another constituent noted is the refrac- 

 tive substance connecting the heterocysts to vegetable cells, called 

 1 ' Verschlusskorper ' ' by Kohl. 



We will now take up the consideration of these various struc- 

 tures more in detail. 



(a) THE NUCLEUS. 



It has previously been mentioned that much controversy has 

 already arisen among workers on the cytology of the Cyano- 

 phyceae over the question whether or not these organisms possess 

 a nucleus. The answer to this question must necessarily depend 

 upon the standard of judgment. If we use as a standard the 

 well differentiated structure in the flowering plants known as the 

 nucleus, with its well defined membrane separating it from the 

 surrounding cytoplasm while in a resting condition, and with its 

 intricate method of division, whereby its most highly organized 

 substance, the chromatin, is handed down to the daughter nuclei 

 in exactly equal quantities, then there is no nucleus in any one 

 of the forms upon which I have thus far worked. This criterion, 

 if rigidly held to, would eliminate a number of structures which 

 have hitherto been considered as nuclei in the lower forms of life, 

 and even in certain cells of highly organized animals and plants. 

 But if it be conceded that these structures last referred to are 

 nuclei, then there is a nucleus in the Cyanophyceae, clearly and 

 unmistakably. The writer's conclusion is based upon the follow- 

 ing evidence. First, the appearance of the nucleus in the living 

 cell ; second, its reaction to stains ; and third, its chemical nature. 

 There is present in the center of the cell of the Cyanophyceae a 

 small refractive structure, well defined in some species and much 

 less so in others, and quite constant in outline in a given species. 

 In order to detect the presence of nuclei in doubtful cases and to 

 make a more careful study of the parts and their behavior, stain- 

 ing is necessary. By this treatment it is found that certain parts, 



