14 Alga 3 



In the Desmidiaceae, in which the cells generally exhibit a more 

 or less deep median constriction, division is only in one direction, 

 and it is brought about by the development of two new half-cells 

 (or semicells) between the old halves. So that each of the indi- 

 viduals formed after one division consists of an old and a new 

 half-cell. In many unicells the division is accomplished by the 

 formation of daughter-cells within the mother-cell. The daughter- 

 cells are rejuvenized and metamorphosed portions of the original 

 mother-cell and are enclosed in the old wall of the mother-cell. 

 Sometimes the daughter-cells are set free by the bursting of the 

 wall of the mother-cell, but it often happens that the old wall of 

 the mother-cell swells up and remains as an outer, wider coat to 

 the daughter-cells. Successive generations of cells are produced 

 in the same manner inside the enlarged walls of the mother-cells 

 until quite a colony is formed inside the swollen cell-wall of the 

 original mother-cell (e.g. many Protococcoideae and Chroococcaceae). 

 In many types of lower Algae there is no definite line to be drawn 

 between this so-called free cell-formation and the ordinary vege- 

 tative division 1 . In certain of the Protococcaceae two or four 

 daughter-cells arise in a mother-cell, and at the time of their 

 escape from the parent-cell they possess the exact form and external 

 peculiarities of the parent ; these have been termed autospores. 



Increase of cells occurs in the filamentous Algae by new 

 divisions, the septa being always transverse to the length. In the 

 GEdogoniaceae the method of cell-division is somewhat specialized 

 and a description of it is given under the family. 



On injury to the filamentous coenocytic Algae septa usually 

 appear cutting off the injured part. The injured portion soon dies 

 away, and if it happens to be in the median part of a filament, 

 two filaments are thus set free. This occurs frequently in 

 Vaucheria, and if the injuries to one filament are numerous, all 

 the injured parts are sometimes cut off, the intermediate uninjured 

 portions developing into new filaments on being set free. 



The reproduction of freshwater Algae is brought about in a 

 great variety of ways, most of the plants exhibiting both an 

 asexual and a sexual mode of reproduction. 



Asexual reproduction. In some cases special non-motile 

 cells develop into cysts or unicellular gemmae (e.g. Zygnemaceae? 



1 Chodat in Ann. Bot. 1897, p. 102. 



