240 PLANT PHYSIOLOGY 



stretched and swell, but when dried out they go back to their 

 original shape. 



In imbibition we have to deal with the same forces that are 

 present in the phenomena of solutions on one hand and capillarity 

 on the other. Only here the two forces are much more nearly the 

 same. The water is attracted with an enormous force resulting 

 in very large pressures. It has been estimated that two cover 

 glasses an inch square require several hundred atmospheres (2,000) 

 to hold them together so firmly that no water can enter between 

 them when placed along the edge. The forcing apart of the bones 

 of the skull by inserting swelling peas and then adding water is a 

 common practice in anatomical work. Boats have been known 

 to split apart when leaks permitted the entrance of water to a 

 cargo of rice or beans, as in the case of the Hapag motorship, 

 Rhineland, in 1926. 



Although, when water is imbibed, there is a change of volume 

 owing to the swelling, the resultant volume is less than the sum 

 of the water and the imbibing tissues. This means that there 

 has been a compression along with the swelling. The result is 

 the emission of heat, which can be easily measured. 



What are the particles of the cell wall which are forced apart 

 by the entering water? Some have thought them to be the mol- 

 ecules of cellulose, but others, led by Nageli, have considered the 

 particles to be groups of molecules which he called micellx. These 

 he thought were the units which played the chief structural and 

 physiological roles in the cell wall. 



In conclusion, then, it must be borne in mind that imbibition 

 and osmosis are the important processes which determine the 

 entrance of water into the cell. The former is a phenomenon 

 allied to capillarity and solution, and in plants is chiefly con- 

 cerned with the dead tissues like the cell walls. Osmosis, on the 

 other hand, is a diffusion phenomenon; it plays the more important 

 part, and is concerned with the transfer of substances into and out 

 of the living cells. 



QUESTIONS 



1. Apply the principles of osmosis to (a) the opening and closing of stomata 

 and (b) the wilting and recovery of plant tissues. 



2. Why do red beets which are boiled lose more of their coloring matter 

 than unboiled ones? 



