RHYTHM 509 



life of the protoplasm is intimately bound iip with such a motile 

 condition. Manifestations of this motility have from time to 

 time been noticed above ; especially we may again allude to the 

 regulation of turgescence, which we have seen to be one of the 

 properties of vegetable living substance. 



In many oases it can be noticed that these automatic move- 

 ments set in with a certain definite intermittence, recurring, 

 so long as conditions are constant, at- regular intervals. The 

 pulsating vacuoles already alluded to may be instanced in 

 this connection. Such regularly intermittent actions may be 

 described as rhythmic and the intermittence spoken of as 

 rhythm. 



WheH we look back upon the various functions we have 

 examined we find evidence in them that rhythmic changes in 

 the protoplasm are very widely met with in the hfe of the 

 vegetable organism. Very conspicuous instances of it are 

 afforded by certain movements often exhibited by the leaves of 

 particular plants. Perhaps the most familiar of these is the so- 

 called telegraph plant, Desmodium gyrans. The leaves are 

 ternate, the terminal leaflet being very large in comparison 

 with the two lateral ones (fig. 1195). If the plant be watched 

 while exposed to a suitable temperature, the lateral leaflets are 

 found to move up and down on the racliis, sometimes passing 

 through an angle of 180°, and 1 vi'isting slightly as they move. 

 They thus describe a kind of ellipse, the duration of the move- 

 ment being about two minutes. Many other instances of a 

 similar kind are known, the Leguminosie furnishing many 

 examples. All of them do not exhibit them with the same ease, 

 as they are much interfered with by the changes in position 

 caused by external stimulation. They can often be made 

 evident by keeping the plant either in the dark, or exposed to 

 light of uniform intensity. Darkness, however, if too prolonged, 

 results in the cessation of the movements, as these can only 

 take place while the plants are in a condition of phototonus. 

 The mechanism of the movement in most of these cases is the 

 varying turgescence of pulvini at the bases of the leaflets. The 

 alterations in this turgescence are the expression of rhythmic 

 changes in the protoplasm of the cells. 



The same rhythmic changes have place in all young growing 

 organs, leading to the variations in turgidity that cause the 

 movements of nutation and oircumnutation which accompany 

 growth. 



The nyctitropic movements we have discussed may be quoted 



