7 1 2 THE AMERICAN NA TURALIST. [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 algae 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 fibrillse 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. 



