712 THE AMERICAN NATURALIST.  [VOL. XXXIX. 
noids and caryoids, should have been displaced by the much 
simpler and apparently homogeneous plastids. 
A comparative study of chromatophores and plastids from 
the point of view of their evolutionary history is much to be 
desired and such research would necessitate extensive studies 
among the lower groups of alge and especially in the Proto- 
coccales. Such studies would involve far more than the general 
morphology of the chromatophore and plastid. The structure 
and activities of the pyrenoid are a very important subject as 
shown by the investigations of Timberlake on Hydrodictyon 
and nothing is known of the function of the caryoid. A de- 
tailed investigation of the chromatophore or plastid throughout 
ontogeny is yet to be made. 
The Cytoplasm.— There is no region of the plant cell whose 
structure is more varied and as little understood as that pre- 
sented by the cytoplasm with its diverse conditions. We have 
throughout these papers held to the classification of Strasburger 
that the cytoplasm may be separated into two forms: kinoplasm 
and trophoplasm, which show certain structural peculiarities and 
are characterized by very different forms of activity. While it 
must be acknowledged that kinoplasm and trophoplasm are very 
similar in certain regions of the cell and at certain periods of 
the cell history, still the distinctions are in general clearly 
marked. 
Kinoplasm is homogeneous in structure, either minutely 
granular or consisting of delicate fibrillæ composed of very 
small granules placed end to end. The homogeneous condition 
is characteristically shown in the three forms of plasma mem- 
branes which cytoplasm places between itself and external or 
internal surface contacts. The three membranes are: the outer 
plasma membrane, the nuclear membrane, and the vacuolar 
membranes. They are certainly closely related and probably 
identical in structure and appear to be the natural expression of 
protoplasm to contact with a fluid (water) medium. The fibril- 
lar condition appears during mitosis and serves important func- 
tions in the mechanism (spindle) through which the chromosomes : 
are distributed and in most of the higher plants determines the 
position of the cell wall that is generally formed with each 
nuclear division, 
