No..466] ©.» STUDIES ON. PLANT CELL.— VII. $47 
. 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. Anda 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 aérial or terrestrial conditions. Our 
attention has been chiefly centered on the structure of the pro- 
toplast and the morphology and behavior of its parts. Weare 
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 
Mirotic PHENOMENA. 
Our brief descriptions in Section II (Amer. Wat., 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, 7. 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- 
