No.' 466.] STUDIES ON PLANT CELL.— VIII. 717 



There are many forms, particularly among the lower plants, 

 where studies on the processes of wall formation are sure to 

 throw much light on the fundamental problems which we have 

 discussed. And a particularly interesting study might be made 

 of the evolutionary history of the cell wall among the thallo- 

 phytes and_ in the modifications introduced when plants pass 

 from aquatic habits to aerial or terrestrial conditions. Our 

 attention has been chiefly centered on the structure of the pro- 

 toplast and the morphology and behavior of its parts. We are 

 likely soon to give more study to the carbohydrate membranes 

 and walls and this subject is likely to be very fruitful for inves- 

 tigation. 



3. Some Apparent Tendencies in the Evolution of 

 Mitotic Phenomena. 



Our brief descriptions in Section II (Amer. Nat., vol. 38, p. 

 431, June, 1904) of the various kinoplasmic structures developed 

 during mitosis in different groups of plants brings up the prob- 

 lem in their relationships to one another, i. e., the evolutionary 

 tendencies in the differentiation of mitotic phenomena. We 

 have seen that the thallophytes present an especially diverse 

 assortment of kinoplasmic structures associated with the spindle 

 and its method of development. The spindle fibers, whether 

 formed within the nuclear membrane (intranuclear) or arising 

 from without (extranuclear), are associated with centrosomes or 

 centrospheres to form asters in a number of well known types 

 as Stypocaulon, Dictyota, Fucus, Corallina, certain diatoms, the 

 ascus, and the basidium. Centrospheres are found in certain 

 phases of the life history of liverworts as in the germinating 

 spore of Pellia. A second type of kinoplasmic structure resem- 

 bling in certain features the aster but with some fundamental 

 differences has been termed the polar cap. The polar cap is an 

 ill defined region of kinoplasm, generally larger than a centro- 

 sphere and without clear boundaries, which forms a region for 

 the insertion of spindle fibers. Polar caps are well illustrated 

 in the mitoses of vegetative tissues and meristematic regions, 

 especially among the higher plants (pteridophytes and sperma- 



