538 PHYSIOLOGY OF GROWTH. 



stalk may sink and the stimulus may overflow to other leaves. 

 When the passage of the stimulus has ceased, and no further 

 movements are evoked, the leaves return again after a few minutes 

 to their position of rest. 1 



1 See Pfeffer, Physiologische Untersuchungen, 1873, and Sachs, Lectures on 

 Plant Physiology. Haberlandt's contributions I do not agree with in all 

 respects (Das reizleitende Gewebesystem der Sinnpflanze, Leipzig, 1890). 



203. Further Movements of Variation Evoked by Shock and 



Contact. 



The leaflets of Oxalis Acetosella, a plant frequently to be met 

 with in damp woods, are provided with pulvini, and react to 

 contact or shock. If the main petiole is agitated, the irritation 

 movement of the leaflets (a depression) can be clearly followed, 

 but many shocks must be communicated to the main leafstalk in 

 order to bring about the maximum depression of the leaflets, 

 whereas in Mimosa the full movement is brought about by a 

 single slight shock. 



The lobes of the stigma of Martynia (Gresneracese), as I have 

 frequently found, are very irritable. If the inside of the stigmatic 

 lobes is touched, they at once lay themselves together. The lobes 

 of the stigma in Mimulus (e.g. M. cardinalis) also react to con- 

 tact. 



The five filaments in the CynareaB are attached at their lower 

 end to the corolla tube. The anthers are united to form a tube 

 through which the style grows. When the pollen is ripe the 

 filaments are irritable, and if not stimulated appear curved, convex 

 side outwards. If stimulated by shock or contact they straighten, 

 at the same time shortening. Pulvini the filaments do not possess ; 

 but the whole parenchyma surrounding the axile vascular bundle 

 is irritable, and when subjected to stimulation loses considerably 

 in expansive force, owing to escape of water, and so is brought 

 about the contraction. To satisfy ourselves that the filaments of 

 the Cynareee are irritable, we may experiment with the flowers of 

 Centaurea jacea. We isolate single flowers from the capitulum, 

 cut across the corolla, filaments, and style, somewhat above the 

 point of insertion of the stamens, and fix the separated sexual 

 apparatus on a cork by means of a pin. It is then placed in a 

 moist atmosphere under a bell-glass. When the preparations 



