262 University of California Publications. [Botany 



mon type. It is the belief of the writer that when all the species 

 of the Cyanophyceae are investigated cytologically, a complete 

 and intelligible series of nuclear differentiations with but few, if 

 any, gaps will be discovered, beginning with a very simple form, 

 and finally diverging into several types which will be found to 

 join on to various species of Chlorophyceae by almost impercept- 

 ible degrees, thus closing up the gap which seems to exist between 

 these two groups of plants. 



While it is not the intention to attempt in this paper any- 

 thing like a phylogenetic arrangement of the species of the group, 

 a sufficient number and variety of forms have been worked upon 

 to note the existence of at least three quite well defined types of 

 nuclei. Furthermore, a sufficient number of forms have been 

 worked out to warrant the conclusion that we have in this group 

 of plants a series of nuclear modifications showing how the nu- 

 cleus of the higher plants may have originated. These three 

 types are based upon the degrees of differentiation of the chro- 

 matin, and its behavior during cell division. They are here desig- 

 nated as the Diffuse type, Net-karyosome type, and Primitive 

 Mitosis type. The third type may be considered as having arisen 

 from the. first by a slight modification. 



Judging from the species I have worked upon, and the mor- 

 phological relation of these to other species in the group, the 

 diffuse type seems to predominate, and probably represents the 

 most primitive nucleus in the group. We will turn to its consid- 

 eration first. This type is characterized by having the chromatin 

 distributed throughout the nucleus in an indefinite way, in the 

 form of thin plate-like or small angular pieces, or more or less 

 branched and knotty thread-like masses, according to the species 

 and to the shape and size of the cell in which it is located ; or even 

 a combination of these may be found in the same species. In 

 whatever shape the chromatin is found, the essential character- 

 istic is its quite equal distribution throughout the nucleus. It is 

 to be found in the homocysted group, e.g., in Oscillatoria, Lyng- 

 bya, Phormidium, Symploca, etc. Within this type may be found 

 quite a continuous series, showing how a very primitive nucleus 

 becomes modified by gradual steps in the direction of the nuclear 

 structure found in the higher plants. 



