SECT. II 



PHYSIOLOGY 2S5 



torsion comes about by a difference in length between the middle 

 line and tlie angles ; all the latter are of equal length. 



Ways in which Curvatures are produced. — In the production of 

 curvatures we are always concerned, as has just been shown, with 

 changes in the dimensions of an organ due to unequal lengthening or 

 shortening. In bringing about these changes in dimension the follow- 

 ing means are employed by the plant, 



1. Growth ; this can only lead to elongation. 



2. Osmotic pressure. This can effect an elongation or a shortening 



according as it is increased or diminished. 



3. Variations in the amount of water in the cell- wall or in 



dead cells. These also can efl'ect eitlier elongation or 

 shortening. 

 According to the means employed in altering the dimensions, the 

 curvatures of plants may be divided, into guowth-cuuvatukes, 



VARIATION MOVEMENTS DEPENDING ON TURGESCENCE and HYGROSCOriC 



MOVEMENTS. Since growth and osmotic pressure are vital phenomena, 

 i.e. are essentially influenced by the living protoplasm, they will be 

 treated below along with the locomotory movements Avhich are 

 dependent on the living substance of the plant. The hygroscopic 

 movements on the other hand are not vital phenomena ; they occur 

 in dying or dead organs and are exclusively brought about by 

 external factors. The protoplasm oidy plays a part in these move- 

 ments in that it has led to sucli a construction of the organs that 

 changes in the amount of water present produce curvatures and 

 not a simple change in length. 



II. Hygroscopic Movements 



Two quite distinct types of movement are included in the 

 hygroscopic movements. In the first, which are termed imbibition 

 MECHANISMS (^^), the cell-walls increase in size on swelling or contract 

 on shrinking. 



The swelling or shrinking depends on the fact that the water of 

 imbibition is not contained in cavities like those in a porous body 

 (such as a sponge or a piece of plaster of Paris) that contain the 

 capillary water, but in being absorbed has to force apart the minute 

 particles of the cell-wall. Conversely these particles approach one 

 another again when the imbibition-water evaporates. When on 

 different sides of an organ there are unequally well-developed layers, 

 or layers that swell with unequal rapidity, or when opposite layers 

 differ in the direction of their greatest extension on swelling, 

 curvatures must take place every time the organ is moistened or dries. 

 Though here Ave are dealing with purely physical phenomena, they 

 may possess great importance for the plant. 



