PHYSIOLOGY 489 



the base, as happens when a bee visits the flower. Many other cases 

 might be cited of similar character, all having to do with the pollina- 

 tion of the flowers. 



Shock. The best-known case of rapid response to shock is the 

 " Sensitive plant," Mimosa pudica. The closely related " Sensitive 

 brier" (Schrankia), of the Southern states, behaves in the same way. 

 A very slight shock is enough to cause the dropping of the petioles 

 and folding together of the leaflets (Fig. 460). A remarkable feature 

 is the transmission of the stimulus to leaves situated a long distance 

 from the part stimulated. As already explained, the movements 

 are caused by loss of water from the cells of the pulvinus, which 

 thus loses its turgidity, and causes the leaf to drop. The vascular 

 bundles serve to transmit the stimulus, the cells especially concerned 

 being in the phloem. 



Wind. The swaying of plants, due to the wind, exercises a 

 powerful effect upon the development of the tissues, both of the stem 

 and roots, which are developed as they are required to withstand the 



Fio. 460. Mimosa pudica. The leaf at the right in its normal position, the one at 

 the left contracted by shock. (After BAILEY.) 



strains to which they are subjected. The mechanical contrivances, 

 thus developed in plants, are extremely perfect. Some of our most 

 important constructive materials, wood and vegetable fibres, are 

 developed by plants in response to the necessity for resisting these 

 strains. 



Similarly, some water-plants are subjected to great strains, espe- 

 cially those which are exposed to violent surf. Some of the large 

 kelps are admirable examples of the way in which there have been 

 developed means of attachment, and tissues of extraordinary strength 

 and flexibility to withstand the tremendous strains to which they 

 are exposed. 



Water as a Stimulus (Rheotropism) 



Water is a necessity to all plants, and it is not strange that certain 

 organs, especially roots, are influenced in the direction of their growth 

 by the presence of water. We have already spoken of the move- 

 ments of the plasmodia of Slime-rnoulds, against a current of water, 



