470 PLANT GROWTH AND PLANT COMMUNITIES 



which elaborated products may be, and no doubt are, released into the 

 cytoplasm. 



Pinocytosis and its possible role in activated transport. It has been 

 a constant source of wonder as to why ions accumulated in vacuoles 

 should ever be withdrawn from the living cell, or even from the 

 vacuoles into the cytoplasm. Isolated cells, as of roots, having absorbed 

 such solutes, hold them so tenaciously that they release them to the 

 outer solution only if they are almost killed. But in the intact plant 

 body, movement from cell to cell and organ to organ is obviously 

 relatively easy. The phenomenon of pinocytosis, studied in amoebae by 

 Holter and others (1959), is highly suggestive here. The outer surface 

 of the protoplasm, especially when in contact with the suitable but 

 concentrated solution, throws itself into pseudopodial folds, and these 

 become vesicular, enclosing a minute canal along which injected fluid 

 and C^^-labeled solutes may pass into the cytoplasm ( Figure 7 ) . Work- 

 ers on pinocytosis now see the process as of more significance because 

 of the resultant injection of dissolved substances than because of the 

 water which gave the process its name originally (Holter, 1959). In- 

 deed, Holter (p. 537) now sees this newly emphasized mechanism "in 

 relation to the whole great problem of active uptake and transport of 

 substances by cells." 



The presence of a cell wall in plants precludes the observation of 

 this phenomena at the outer membrane surface, in contact with the 

 more dilute external solutions. However, the vacuolar surface has long 

 been known to be a seat of movement, and protoplasmic strands, which 

 often traverse the vacuole, may undergo "make and break" ( see Figure 

 8). It is conceivable, therefore, that whereas the entry of solutes into 

 the vacuole may be negotiated along a discrete system of canals (the 



Figure 7. Diagram to show 

 pinocytosis of inorganic salts 

 in amoebae. (From Chap- 

 man-Andresen, 1958.) 



