280 COMPENDIUM OF GENERAL BOTANY. 



the movements of the lateral leaflets of Hedysarum gyrans as 

 well as the movement known as " rectipetality," discovered by 

 VoCHTlNG. This latter movement manifests itself in an effort of 

 geotropicallj curved shoots to become straightened when gravity 

 is counteracted by means of the clinostat. 



External causes, such as temperature and light, modify the 

 various autonomous movements.' 



D. Irritable Movements. 



What has been said in regard to the effects of gravity, light, 

 temperature, etc., is to be applied herein so far as movements 

 are concerned. All of the external influences of the plant- 

 organism are stimuli in the wider sense ; hence the heliotropic, 

 geotropic, and also the nyctitropic movements are irritable move- 

 ments in the loider sense. We usually recognize irritable move- 

 ments in a narroiver sense, caused by mechanical shock, or contact, 

 accompanied by growth or without growth. 



As an example of irritable movement without growth we will 

 discuss Mimosa pudica ; as an example of irritable movement 

 with growth we will discuss the behavior of tendrils. 



Mimosa. 



Our insight into the principles of the mechanics of irritable 

 and related movements due to stimuli ceases where all deej) and 

 far-reaching investigations of life-processes cease, namely, with 

 the question. Why does the plasm of living cells become changed 

 in response to certain stimuli ? The result of this plasmic be- 

 havior — in this special case the sudden passage of Avater from the 

 cell-lumen through the primordial utricle into the intercellular 

 spaces and to the exterior — may then be considered and explained 

 mechanically. We owe our more exact knowledge in regard to 

 these conditions and relations to Brucke and Pfeffer. The 

 mechanical principles underlying the movements in Mimosa 

 also apply to the movements in the stamens of Cynarece. In 



' Incidentally it may be noted that there is no such contraction and relaxation 

 in the vegetable tissues as is seen in the muscles of animals. Active shortenings 

 or contractions are, however, not wanting ; as has been mentioned, increase in 

 turgor will cause root parenchyma cells to become shortened. 



