CHiapter XIV 



— 153 



The Vacuolar System 



it possible to follow their entire development without using vital 

 dyes. The large vacuoles of mature cells, instead of staining dif- 

 fusely without precipitation or forming only a very few precipi- 

 tates, may behave differently. Very often, as for example, in the 

 epidermal cells of Iris germanica, the vital dyes do not stain the 

 vacuolar sap and only produce in the vacuole deeply stained bodies 

 showing Brownian movement or else bring about, at the same 

 time, both a diffuse coloration of the vacuolar sap and the pro- 

 duction in the sap of colored precipitates. 



- v*- 



• Vf f 



i^ 



••• 



F^G. 97. — Pea root vitally stained with neutral red. 

 1-4, meristem; numerous small filamentous vacuoles, uniformly 

 stained. 5-7, differentiating cells; swelling and fusion of small 

 vacuoles, whose colloidal substance is precipitated by the dye 

 as deeply stained bodies showing Brownian movement. 



So the vacuoles seem to be at first small elements composed of 

 a very concentrated colloidal solution which, by taking in water, 

 gradually swell and coalesce during the period of differentiation 

 of the cells, and become large vacuoles containing an extremely 

 dilute colloidal solution. This progressive dilution of the vacuolar 

 colloidal solution is made evident by the fact that neutral red, 

 which at first gives the vacuoles a deep homogeneous color, stains 

 them only weakly when the cells have attained a certain degree of 

 dilution but generally brings about a precipitation of the colloid 

 as deeply stained bodies which show Brownian movement. The 

 precipitation is more or less copious depending upon the nature 

 and concentration of the colloid. 



When the vacuoles have come to the end of their development, 

 they may, in the presence of vital stains, stain weakly and homo- 

 geneously without showing any, or at most very few, precipitates, 

 as in the roots of barley and wheat (except for the cells of the 

 root cap). In other, more frequent cases, vital dyes bring about 

 the formation of numerous deeply stained precipitates, which show 

 Brownian movement, at the same time that they stain the vacu- 

 olar sap diffusely (vacuoles in the epidermal cells of Iris germa- 

 nica) . In still other cases, the vital dyes at first cause only colored 

 precipitates. These later fuse and may dissolve in the vacuolar 

 sap which then becomes diffusely colored. 



