Vol. 2] Gardner. — Cytological Studies in Cyanophyceae. 267 



division and ingrowing cell-wall. These are not rare cases, but 

 may be found in great abundance in any collection of actively 

 growing plants. 



The division of the nucleus in all the filamentous^ species 

 studied, including both homocysted and heterocysted forms, is 

 direct, or amitotic. I have never observed in any such species 

 the breaking up of the chromatin into a definite or an indefinite 

 number of chromosomes either in the form of rods or of granules. 

 After cell division the chromatin in each daughter cell (practi- 

 cally one-half of the original amount in each cell) undergoes no 

 essential change, retaining the same general structure as before 

 division, and simply increasing in quantity ; in many cases it does 

 not wait to attain its normal size before beginning another or 

 even two divisions. Although the chromatin of the spores of 

 various heterocysted species unites into network in a similar 

 manner as does that of the Net-karyosome type, its division is 

 the same as that of the vegetative cells, that is amitotic. The 

 whole mass of network is divided into two equal parts, the sepa- 

 ration beginning at the margin and proceeding toward the center 

 until the whole is divided. Fig. 20 represents a nucleus com- 

 pletely divided and the cell-wall grown across, dividing the orig- 

 inal cell into two equal parts. Spore division does not always 

 take place in this manner. Usually several divisions may be seen 

 in progress at the same time. 



The division of the nucleus in the Net-karyosome type may 

 also be said to be amitotic. No spireme, spindle, or chromosome 

 formation is present. We have in the species in which this form 

 of nucleus is present a different procedure in division from that 

 which is usually found in other organisms. The mature cell of 

 the Dermocarpa may be several hundred times larger than the 

 original young cell; at the same time there is but one mass of 

 chromatin all united into a definite network, or in other words, a 

 single very large nucleus. The chromatin in this large network 

 breaks up simultaneously into a large number of small parts 

 about equal in size. These become the centers of new cells which 

 are now formed by the simultaneous laying down of new cell- 

 walls around each mass of chromatin and its surrounding cell- 

 substance. The mother cell-wall now ruptures and the whole 



