28 PROTOPLASM 



cells. Not only the similarity in external resemblance of the 

 Golgi apparatus and reticulate plant vacuoles but also the fact 

 that silver, used as a fixative, is reduced in both suggests that 

 the two are identical structures. 



The filamentous vacuole is a remarkable structure. It may be 

 of such length as to resemble a skein of yarn (Fig. 18). No pio- 

 neer worker in cytology would ever have recognized such a struc- 

 ture as a vacuole. Holmgren, in 1903, first described structures, 

 since known as Holmgren canals, which are similar to if not 

 identical with filamentous vacuoles. Earlier investigators missed 

 the threadlike plant vacuoles, because these structures show up 

 only when the cell is killed in basic (alkaline) fixatives. Cyto- 

 logical technique has been carried on mostly with acid fixatives, 

 because they are the best for revealing chromosomes on which 

 the interest of biologists has primarily centered. Short, fila- 

 mentous vacuoles resemble rod-shaped mitochondria, which has 

 led to the statement that there is probably an identity between 

 the two (as in the case of the Golgi apparatus of animals and the 

 reticulate vacuole of plants) but cytologists believe that mito- 

 chondria are not vacuoles. 



Some vacuoles maintain a definite shape; others change. 

 I. W. Bailey describes threadlike vacuoles as arising from 

 spherical ones through active streaming of the protoplasm. In the 

 hange which takes place from the one extreme to the other, the 

 vacuole may, in an intermediate stage, resemble a string of beads. 

 When vacuoles are of maximum size they become huge cavities 

 occupying nearly all of the volume of a cell, only a thin layer 

 of protoplasm remaining. 



The contents of vacuoles vary as much as do their shape 

 and size. Probably all typical vacuoles contain salts and sugars 

 in aqueous solution. It is very likely that they also contain 

 some protein matter. Aleuron grains (minute albuminoid 

 granules) have been described as present in some vacuoles, and 

 tannin in others. Certain fatty substances such as lecithin may 

 also be present. More than this cannot be said. It is not always 

 easy to distinguish vacuoles by their contents. I. W. Bailey 

 has shown that two vacuoles within the same plant cell may be 

 identical so far as the human eye can detect, yet one stores tannin 

 and one not. 



The function of vacuoles is probably also varied. Some 

 extraordinary uses have been ascribed to them, but the three 



