200 FRESH-WATER ALG^ OF THE UNITED STATES. 



circular slit appears, and at the same time a partition forms, so that from the 

 mother-cell are developed a small apical and a large basal daughter-cell. The his- 

 tory of the former of these is simply one of growth, as regards the main axis. It 

 increases in size but does not give origin to new cells. All such cells are formed 

 out of the basal daughter-cell, which, as already described, divides into a new 

 apical and basal cell — the apical only to grow in the main filament — the basal to 

 divide anew. It is always the basal cell that undergoes division, throughout the 

 whole life-history of the plant, one cell alone contributing to the growth of the 

 main filament. The filament thus formed bears upon its distal end the hair which 

 grew upon the original spore-cell, and this hair is, save only the basal cell, the 

 oldest part of the filament. The ceU upon which it rests is the next oldest, the 

 next to it in position, the next in age, and so on (from older to younger) down to 

 the basal cell, the oldest of all, lying next to the latest born. 



Although the cells of the main filaments do not contribute to its development, 

 yet it is from them that the lateral branches are formed. The production of a 

 branch begins by the appearance of a clear space near the apex of the cell, but this 

 clear space is placed, not exactly at the apex, but a little to one side. It soon becomes 

 distinctly conical, enlarges, bursts through the old cell-wall, is cut off by a cellulose 

 partition from its parent, and develops into a hair similar to that first formed, but 

 placed at an angle to the long axis. It is remarkable that the opening for the 

 exit of the growing hair occurs, not by a circular transverse slit, but by a longi- 

 tudinal one, the two halves of the old cell-waUs separating as the little cone pushes 

 its way between them and persisting as a sort of sheath to its base. When the 

 hair is perfected the cell from which it grew undergoes division in the usual way, 

 save only that the cutting off of the old wall is done obliquely instead of trans- 

 versely, so that the partition is oblique instead of horizontal, and the new cell 

 grows at an angle to the old, instead of in the line of its axis. The new cell, 

 consequently, is the starting point to a branch at an angle to the main filament. 

 This branch, like the main filament, grows only by the repeated divisions of its 

 primal basal cell, and bears aloft its seta. Secondary branches may arise from it 

 precisely in the way that it arose from the parent stem, and thus at last is formed 

 the bushy plant of the BuTbochceteoe. 



The zoospores closely resemble those of the CEdogoniecE, and are oval or glo- 

 bose masses of chlorophyllous protoplasm, with a transparent space at the smaller 

 end, surmounted by a crown of cilia. Their mode of formation and Avhole life- 

 history are also similar to that of the" CEdogoniece zoospores, up to the time when 

 in their germination they begin to produce new cells. 



Sexual reproduction amongst all the known BulbochceiecB is similar in its general 

 aspect to that seen among the gynandrous CEdogoniece, but differs considerably in 

 detail. The oogonia are mostly formed in lateral branches. Their position in these 

 branches varies in the various species. 



Since any cell from the next to basal to the most distal of all crowned with the 

 terminal seta may be converted into a oogonium, according to Pringsheim, the 

 cell which is to form the oogonium arises in the usual way, by the division of a 

 cell into two daughter-cells. The new daughter-cell, which is to develop into the 



