THE MOVEMENTS AND SENSITIVITY OF PLANTS 341 



tube filled with water, irritation nevertheless will be transmitted 

 through this bridge of water. This has given rise to a theory that 

 it is not excitation which is transmitted, but a hydrostatic pres- 

 sure, similar to the transmission of a wave of compressed air in the 

 tube of an air hammer. Recent investigations by Ricca, how- 

 ever, have somewhat contradicted this theory. They have con- 

 firmed the fact that irritation is transmitted through dead sections 

 of tissues and through glass tubes. Still, Ricca believes that a 

 hormone originates in the stimulated cells and that its transmission 

 is not through the living cells, but through the vessels. These 

 findings, however, require further confirmation. 



Seismonastic sensitivity is present also in a few other plants 

 though in a much lower degree. It is exhibited by the leaflets of 

 woodsorrel and other representatives of the Oxalidaceae and by 

 some of the leguminous plants, including the common locust. It is 

 found in the leaves of certain insectivorous plants, particularly in 

 Dionaea. The leaf blades of the latter consist of two halves, 

 joined by a sort of hinge, which rapidly shuts when the peculiarly 

 sensitive hairs distributed in the central portion of each half are 

 touched (Fig. 95). It should be noted in passing that the seis- 

 monastic movements of insectivorous plants are almost the only 

 instances of movements of this type. Their ecological importance, 

 the catching of insects, is quite well known. The role of the vari- 

 ous other movements, those of Mimosa included, are, however, not 

 clear. At present one has to be satisfied with rather doubtful 

 explanations, such as, for example, that the folding of the tender 

 leaves of Mimosa caused by the first drops of rain protects them 

 from being injured by a tropical shower, or that these move- 

 ments scare caterpillars and other animals which feed on their 

 foliage. 



Seismonastic sensitivity is also shown by flowers of certain 

 plants. In many flowers of the family Compositae, such as the 

 cornflower, the stamenal filaments exhibit considerable contraction 

 if touched. The mechanism of this contraction is similar to that 

 found in Mimosa, i.e., a rapid increase in the permeability of par- 

 ticular cells. A touch on the stamens of the barberry causes them 

 to bend rapidly and press against the stigma. In certain plants 

 (Mimulus, Martynia) similar movements of the fringed stigma 

 may be observed. All these movements serve to facilitate a suc- 

 cessful pollination of the plant. 



