268 University of California Publications. [Botany 



mass of small cells floats out. These cells are known as conidia 

 or schizospores. They are non-motile, and when they fall on a 

 suitable substratum, usually on some living plant, they begin to 

 grow and the chromatin mass increases in size, very soon forming 

 another chromatin net as before. 



Of all the forms of nucleus in the Cyanophyceae which I have 

 investigated the Primitive Mitosis type as illustrated by Synech- 

 ocystis aquatilis (Figs. 40-44) approaches nearest to the mitosis 

 of the lower Chlorophyceae. The chromatin unites into a thread 

 in preparation for cell division. This thread is not, however, a 

 very definite one as regards its shape and disposition in the cell. 

 The thread breaks crosswise into a definite number of segments, 

 in this species three, which arrange themselves parallel to each 

 other and to the long diameter of the cell. These break apart in 

 the middle when the new cross-wall is to be laid down. 



In this method of division, as in that of the first method men- 

 tioned, the cell-wall seems to push the segments toward the center 

 in some cases and they are not always divided simultaneously. 

 Fig. 43 illustrates this method. It should be said that the divi- 

 sion of the chromatin thread seems to precede somewhat the in- 

 growing of the cell-wall, and the impression is not produced, as 

 it is in Oscillatoria, that the ingrowing wall simply cuts the chro- 

 matin mass in two. 



It will be seen from the foregoing methods of division of 

 chromatin in the Cyanophyceae that there is not an exact divi- 

 sion of the chromatin into equal parts, but only an approxima- 

 tion thereto, and further that there is no longitudinal splitting 

 of the threads of the chromatin. 



The formation of a constant number of chromosomes and the 

 longitudinal splitting of the same in order to accomplish the 

 exact partition of all of the chromatic elements, the bearers of 

 the hereditary qualities, have constituted a working hypothesis 

 upon which to explain inheritance. It is acknowledged that these 

 plants have been handed down to us from very ancient times, 

 and yet without this complicated karyokinetic process found in 

 higher organisms they go on reproducing themselves in such fash- 

 ion that each succeeding generation resembles the parent one as 

 nearly as may be found elsewhere in the organic kingdom. 



