MORPHOLOGY OF ANGIOSPERMS. 117 



development from the young to the mature stage. These are 

 known as growth movements or nutation. They are especially 

 noticeable in tips of growing branches, which instead of growing 

 in a straight line, move either from one side to the other, or coil 

 or curve about an imaginary axis. This spiral movement is 

 known as circumnutation and is characteristic of twining stems and 

 tendrils, as the hop vine (Fig. 136) and tendrils of Bryonia (Fig. 

 66) . Nutation curvatures are due to unequal growth on two sides 

 of the organ and cease when there is a cessation in growth or 

 when the plant has reached maturity. 



The movements of organs due to external stimuli are usually 

 in a direction which shows a relation to the direction of the stim- 

 ulus, as those produced by gravity and light (Fig. 61), and these 

 movements are of use in bringing the organs into more favorable 

 positions for growth. Stimuli of this kind are spoken of as 

 orienting or tropic. The compound leaves of a number of plants 

 exhibit in addition certain variable and periodic movements, which 

 have their origin in a special mechanism known as the pul\'inis. 

 The pulvinis appears as a swelling on the petiole and consists of 

 parenchymatous tissue which is highly turgid, i.e., full of water. 

 Any stimulus, such as mechanical shock, which causes a differ- 

 ence in the degree of turgidity on two sides, will result in a move- 

 ment of the leaves in such plants as Mimosa, Oxalis and locust. 

 The leaves of Mimo.m pudica, a common cultivated sensitive plant, 

 show a very rapid response to such stimuli, the leaflets folding 

 together and the petiole and petiolules drooping. In other cases 

 there is a change in the position of the leaves following the alter- 

 nations of day and night. During the day the leaflets are spread 

 out freely, but at night or in darkness they droop and fold 

 together. These are spoken of as nyctinastic (nyctitropic) or 

 " sleep movements," and are exhibited by a number of leguminous 

 plants, as clover, bean. Cassia (Fig. 71), and by wood-sorrel 

 {Oxalis Acetosella) and various cultivated species of Oxalis. The 

 leaves of Oxalis as well as of some other plants fold together 

 under the influence of intense light as well as at night or when 

 the amount of light is reduced. Of special interest also are the 

 lateral leaflets of Desmodium gyrans (telegraph plant) which 

 describe curvatures at more or less regular intervals day and 



