CHEMICAL AND PHYSIOLOGICAI, STUDIES. 185 



length is also proceeding an ellipse or spiral ; this is then 

 known as a circanmutation. 



In flat (dorsiventral) organs an upward or downward or side 

 to side movement will be produced by the same variations in 

 growth, and the phenomenon is here known as epiuasty or 

 hyponasty as the case may be. These movements are known as 

 movements of variation. 



So-called nyditropic movements are due to curvatures in organs 

 (leaves) owing to the periodic variations in turgidity of certain 

 cells, which are determined by the altered conditions of growth 

 as night approaches. The cells which cause these movements 

 are usually situated at the bases of the petioles (in leaves), so 

 that a drooping of the organ occurs, or if the organ is a flower, 

 a closing of this. 



The twining or revolving movements of stem-climbers are 

 due to the action of gravity, which causes an increased growth on 

 either the right or left side of the growing internodes of the 

 apex of a shoot. In this manner, either right-handed (dex- 

 trorse) or left-handed (sinistrorse) curves may be produced, 

 which serve to twine the stem round a support. 



In the case of the nodes of grass-haubns, the resting tissue of 

 these parts may be stimulated by the action of gravity, so that 

 the lower aspect of the node exhilnts increased growth ; this 

 causes an upward bending of the haulm. 



Tactile stimuli produce measurable effects upon certain grow- 

 ing organs. At times, especially in certain climbing plants (tendril- 

 climbers), the contact of a resisting bodj, such as a wall, or stick, 

 causes increased growth in those cells of the plant-organ which 

 are remote from the stimulus, and this has the result of producing 

 a curvature which enables the organ (tendril, &c.) to entwine 

 the support. In Drosera, contact stimuli will cause the glandular 

 hairs" of the leaf to curve over and enclose a small particle which 

 has fallen upon them ; and if this particle happens to be of 

 an organic nature (albumen, small fly, &c.), a further action 

 results — viz., secretion of a peptic ferment in certain cells of 

 the leaf. 



In Mimosa pudica, tactile stimuli applied to one of the small 

 pinnse of the bi-compound leaf, causes first of all a drooping of 

 the pinna, then of the lateral leaflet, and finally of the whole 

 leaf. This result depends upon the disturbance of the osmotic 



