Schizogoniales 279 



Order 5. SCHIZOGONIALES. 



This order was instituted (G. S. W. '04) to include the single family of 

 the Prasiolacese, in which the thallus is filamentous, or subparenchymatous, 

 and often expanded into broad sheets. The expanded thalli sometimes arise 

 by the concrescence of the filaments in one plane and sometimes by the 

 regular division of the cells in two directions in one plane. 



These Algae are subaerial, and the expanded thalli are attached to the 

 substratum by rhizoids. 



The order is at once distinguished from the Ulotrichales by the axile 

 chloroplasts and by the division of the cells in two directions in one plane, or 

 even in three directions in young plants. From the Ulvales it is distin- 

 guished by its chloroplasts and by the more regular arrangement of the 

 thallus-cells consequent upon their division only in two directions at right 

 angles. It is not improbable that the Prasiolacese originated from the 

 Protococcales along a line very different from that along which the Ulvales 

 evolved. 



Family Prasiolacese. 



It is possible that all the forms of this family (= the Schizogoniaceae of 

 Chodat, '02, the Prasiolacese of West, '04, and the Blastosporacese of Wille, 

 '09) are referable to the one genus Prasiola Agardh (1821), although there 

 are some reasons for the retention of Schizogonium Klitzing (1843) for the 

 simpler types. The thallus is either a simple unbranched filament or it 

 forms a flat expansion one layer of cells in thickness. The simple filament 

 is the Hormidium-st&te and when several of these filaments fuse laterally 

 they give rise to the Schizogonium-st&te. In the wider expansions the 

 cells usually divide in two directions in one plane resulting in the Prasiola- 

 state. In these flat expansions the cells are frequently arranged in regular 

 groups of four or multiples of four, such groups being as a rule separated by 

 rather thicker walls. In the Hormidium-statQ the cells are shortly cylindrical 

 (fig. 179 A and B), but in the expanded Prasiola they are sometimes 

 angular by compression, and so situated that their long axes are at right 

 angles to the plane of the thallus. Each cell possesses a single nucleus and 

 a central stellate chloroplast. The latter is much lobed and is furnished 

 with one pyrenoid. The cell-wall is strong, hyaline and fairly thick. Most 

 of the expanded thalli are fixed to the substratum by means of rhizoids, 

 which may be developed from any cell of the thallus, although they are 

 mostly found as outgrowths from the more basal cells (fig. 179 G). 



