DIV. ii PHYSIOLOGY 333 



According to the means employed in altering the dimensions, the 

 curvatures of plants may be divided into GROWTH-CURVATURES, 



VARIATION MOVEMENTS DEPENDING ON TURGESCENCE, and HYGROSCOPIC 

 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 which 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 brought about exclusively by 

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

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

 changes in the amount of water present produce curvatures and 

 not a simple change in length. 



A. Hygroscopic Movements 



Two quite distinct types of movement are included in the 

 hygroscopic movements. In the first, which are termed IMBIBITION 

 MECHANISMS ( 95 ), 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 we are here dealing with purely physical phenomena, they 

 may possess great importance for the plant. 



The rupture of ripe seed-vessels, as well as their dehiscence by the opening of 

 special apertures, is a consequence of the unequal contraction of the cell walls due 

 to desiccation. At the same time, by the sudden relaxation of the tension, the 

 seeds are often shot out to a great distance (Euphorbia, Geranium, etc.). This 

 dehiscence on drying is termed XEROCHASY, and is contrasted with the opening of 

 the fruits and dispersal of the seeds in some desert plants when they are moistened 

 (HYGROCHASY). The best example of this is the fruit of Mesembryanthemum 

 linguiforme. The behaviour of the " Rose of Jericho " (Anastatica hierochuntica) 

 is similar. The whole plant when fruiting dries up, and owing to the unequal 

 shortening of the upper and under sides of the branches becomes contracted into a 

 spherical mass. On the addition of water, the plant resumes its original form, its 

 fruits open and shed the seeds which are thus under favourable conditions for 

 germination. With Anastatica some other plants (e.g. Odontospermum) may be 

 mentioned, to some of which the name Rose of Jericho is also applied. In certain 

 fruits not only curvatures but torsions are produced as the result of changes in the 



