36 



FUNDAMENTALS OF CYTOLOGY 



unable to accept this view. If this problem could be definitely solved 

 with the discovery of the exact reasons why some of the minute bodies 

 develop into plastids while others in the same cell or in other organisms do 

 not, we should be nearer to a solution of the more general problem of the 

 real significance of chondriosomes in protoplasmic activity. Researches 

 to date have at least served to foster a more critical interpretation of the 

 effects of fixation. 



Fig. 23. — Cell from inner side of bean 

 pod after treatment with Janus green B. 

 m, mitochondria; O, oil globules; P, poly- 

 morphic plastids, colorless and partially 

 impregnated with chlorophyll (shaded). 

 (After H. Sorokin.) 



Fig. 24. — Cells from young shoot of rye. 

 In the cytoplasm are small granular 

 mitochondria, large, deeply staining plastid 

 primordia, and plastids containing starch. 

 {After J. A. O'Brien, Jr.) 



Vacuoles. — Vacuoles are liquid-filled cavities in the cytoplasm or, very 

 rarely, in other portions of the protoplast (Figs. 25, 26). They are char- 

 acteristic chiefly of plants, where they are conspicuously developed in cells 

 of nearly all kinds. They are also prominent in protozoan cells, while 

 smaller globules staining with neutral red in the tissues of higher animals 

 seem to be of the same general nature. By virtue of their osmotic proper- 

 ties, plant vacuoles function in the maintenance of turgor, which is of 

 importance to metabolic acti\-ity and contributes to the support of 

 herbaceous bodies. Fiu'thermore, they serve as repositories for certain, 

 classes of reserve products and by-products. To what extent such 

 secretory activity involves reactions within the vacuoles rather than in the 

 cytoplasm near by is not well known. The vacuoles in animal tissues play 

 problematic roles. The rhythmic filling and discharge of contractile 

 vacuoles in lower animals and plants appears to be an excretory process, 

 and it is further thought probable that it aids in the regulation of hydro- 



