94 



INTRODUCTION TO CYTOLOGY 



laria (liverwort) they are very large. In the cambium of woody plants 

 they are very abundant and exhibit a surprising variety of form (Fig. 

 48) (Bailey, 1930). As the meristematic cells multiply, the vacuoles 

 are distributed, often after passive division, to the daughter cells. Dur- 

 ing the final growth and differentiation of the cells the vacuoles coalesce, 

 commonly forming a single enormous vacuole which far exceeds the 

 volume of the cytoplasm in which it lies. The vacuolar system of a cell 

 or other protoplasmic mass, whether this system consists of one or more 

 vacuoles, is called the vacuome (Dangeard). 



Much of the difference of opinion regarding the origin and occurrence 



of vacuoles has been due to the fact 

 that many fixing fluids, particularly 

 those containing acetic and chromic 

 acids, may destroy the true vacuoles 

 and greatly alter the appearance of 

 the cytoplasm. Certain fluids con- 

 taining iron or chromium salts pre- 

 serve very well the size and form of 

 the vacuoles but not their membranes 

 (Zirkle), The only trustworthy stud- 

 ies on this subject, as in the case of so 

 many others in the field of cytology, 

 are those which are checked as far as 

 possible by observations on living 

 material. 



As just intimated, the mode of 

 Fig. 48.-Various forms assumed by origin of vacuoles has long been a 



the vacuome in fusiform cambial initials of debated qUCStion. An old and Still 



Robinia. (After Bailey, 1930.) prevalent vicw is that they simply 



arise de novo in the cytoplasm wherever water and certain dissolved sub- 

 stances become abundant enough to form visible droplets (von Mohl, 

 Nageli). Pfeffer (1890), who found that droplets with membranes 

 were formed when granules of asparagin were introduced into protoplasm, 

 favored this view. Strasburger (1898), who regarded protoplasm as 

 mostly alveolar in structure, accounted for the origin of vacuoles by 

 the enlargement and coalescence of alveoles, while Meyer (1912, 1920) 

 classed them as accumulations of ergastic fluid around which membranes 

 are secondarily formed. 



Sharply opposed to this interpretation was that of de Vries (1885), 

 who advanced the theory that vacuoles do not arise de novo but rather 

 from individualized bodies which he called tonoplasts. As the tonoplast 

 secretes cell sap within itself, it gradually enlarges and becomes the 

 vacuolar membrane, which is still referred to as the tonoplast. Since 

 the tonoplast bodies were supposed to multiply only by division, de Vries 



