414 MOVEMENTS. 



through its elliptical orbit in a period of from half a minute to a 

 minute or more, the time var3-ing greatly according to the ex- 

 ternal conditions, but being nearly- uniform under uniform high 

 temperature. The lateral leaflets move independentl}- of one 

 another, one sometimes passing downwards while the other is 

 ascending, but there is no distinct relation between them. 



At the base of the terminal leaflet, the base of the lateral leaf- 

 lets, and the base of the main petiole, are pulvini, to changes 

 in which the several movements are due. 



1070. The cause of autonomic moTements not fully known. As 

 to the cause of the periodic changes in turgescence and asso- 

 ciated growth which give rise to " spontaneous " movements, little 

 is at present known. The fact that in the naked protoplasm of 

 the Plasmodium of the M3-xoni3-cetes the sap cavities exhibit 

 a rhythmical pulsation which is thought to be dependent npon 

 variations in the imbibition power of the protoplasm for water, 

 throws little light upon the ultimate cause which underlies vari- 

 able turgescence in one case and variable pulsation in the other. 

 Although vaiiations in turgescence and associated growth are 

 everywhere observable in joung and still parts of plants, in some 

 instances similar phenomena can be observed, as we have just 

 seen, in specialized organs which are no longer capable of 

 growth. 



1071. DeVries ' calls attention to the fact that organic acids or 

 their salts, as thej- are formed in tissues, have a maikcd eflfeet 

 upon the turgescence of the cells composing the tissue. If these 

 compounds were produced first in the cells on one side of a shoot 

 or other motile organ, and then in the cells next to these, and 

 so on, the phenomena of circumnutation would be exhibited. 

 Its cause will probably be found in chemical processes which 

 cause the osmotic power of the cell-contents to var}'.'^ 



1072. SensltiTeuess. B}- this is meant the capacitj- to react 

 against an irritation ; thus, the root is said to be sensitive to 

 moisture, some leaves to light, etc. But it is usual to employ- 

 the term in a more restricted signification ; following Darwin's 

 cautious definition, " a part or organ maj- be called sensitive, 

 when its irritation excites movement in an adjoining part." ' The 

 iiTitant may be shock, prolonged contact, a light touch, or a 

 chemical agent. 



1 Botanische Zeitung, 1879, pp. 830, 847, and in an independent commnni- 

 cation. 



^ Pfeffer : Periodisehen Bewegungen (1875). 

 8 Power of Movement in Plants, 1880, p. 191. 



