578 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Zoospore Formation in Characium.* — G. M. Smith gives an 

 account of zoospore formation in Characium Sieboldii A. Br., one of the 

 Protococcales. jMature cells of this plant contain 32-64 nuclei, and one 

 or more irregularly shaped pvrenoids. The process of zoospore 

 formation is one of progressive cleavage, the first cleavage planes being 

 transverse and the later ones longitudinal. The cleavage continues 

 until angular uninucleate protoplasts are formed. The pyrenoid does 

 not divide, but disappears during the process. The angular protoplasts 

 become zoospores by rounding up, forming pyrenoids and cilia. After 

 coming to rest the zoospore develops without cell-division into the new 

 plant, but the nuclei increase in number l>y simultaneous division as the 

 cell enlarges. 



'to^ 



Cytology of Pediastrum.f— G. M. Smith describes the cell structure 

 and zoospore formation in Pediastrum Boryanum (Turp.) Menegh., one 

 of the HydrodictyaccEe. The youngest cells of this plant are 

 nninucleate, and each contain one pyrenoid (rarely two or three). 

 Mature cells contain four or eight nuclei and one to three pyrenoids. 

 The nuclei increase in number by sinuiltaneuus division so that the 

 number is always a multiple of two. In structure the resting nucleus 

 differs but little from the nucleus of the higher plants. The pyrenoids 

 are homogeneous in structure and surrounded by curved starch plates. 

 Previous to zoospore formation there is a period of active simultaneous 

 nuclear division resulting in 16, 32, 64, or 128 nuclei within the cell. 

 The zoospores are formed by cleavage. The cleavage is progressive, 

 forming first multinucleate protoplasts and later uninucleate ones. The 

 pyrenoid disappears previous to or during the first stages of cleavisge. 

 After cleavage is completed the nuclei l)ecome dense, and the line of 

 demarcation l)etween the protoplasts disappears, reappearing shortly 

 before the zoospores are liberated. 



Organization of the Colony in Certain Four-celled AlgaB.}— 

 G. M. Smith discusses this subject and describes his experiments. His 

 results are summarized as follows : — The arrangement of the cells in 

 coenobic alg^ may be classified according to the relationships of the cell 

 axes (isoaxial and heteroaxial). Pure cultures or unialgal cultures 

 should be used for studying the variations occurring in any given 

 species. The marked variation described by Grintzesco for Scenedesmiis 

 acufus was not found in the author's pure cultures. Changes in the 

 environment produce changes in the individual cells of the ccenobe 

 (Biaimnetamorphosis), but have small effect upon the special inter- 

 relationships of the cells. The variations in cleavage of the mother-cell 

 are little affected by external conditions. When the autospores are 

 motile they are influenced by external conditions through the influence 

 on the motility of the zoospores. The variations occurring in the 



* Ann. Bot., xxx. (1916) pp. 459-66 (1 pi. and figs.), 

 t Ann. Bot., xxx. (1916) pp. 467-79 (1 pi. and figs.). 



X Trans. Wisconsin Acad. Sci. Arts and Letters, xvii. (1914) pp. 1165-1220 

 (7 pis. and 7 figs, in text). See also Bot. Centralbl., cxxxii. (1916) pp. 173-5. 



