IRRITABILITY 247 



plants has been demonstrated by various authors, * though 

 Sachs's claim that roots are sensitive to contact seems to 

 have been successfully disproved by Newcombe.f New- 

 combe shows that roots do not bend about harmless sub- 

 stances ( e. g. glass rods and splinters of tannin-free wood ) 

 although they do coil around pins, brass wire, and rods 

 made of wood containing tannin or other injurious mat- 

 ters. The bendings which Sachs described were responses 

 to injury ( traumatropic ) rather than to contact.! Studies 

 of the thigmotaxis of plants are very few, if any at all 

 exist, yet the behavior of such motile organisms as the 

 diatoms, Oscillatoria, Beggiatoa, etc., and the relations of 

 zoospores to the solid substances to which they attach 

 themselves, offer objects as interesting as they are difHcult 

 for investigation. 



Something must now be said about the so-called "sensi- 

 tive plants." These respond to a touch, a blow, or even a 

 sudden breath of air, to a drop or stream of water upon 

 the leaves, as well as to changes in illumination, tempera- 

 ture, etc. These plants have compound leaves, the petioles 

 of the leaves and the stalks of the leaflets being supplied 

 with cushion-like enlargements called pulvini, which serve as 

 an articulation between petiole and blade or between petiole 

 and branch. A pulvinus is composed mainly of parenchyma 

 tissue, the cell-walls of which are elastic and readily per- 

 meable to water. The flbro-vascular bundles which, in the 

 petiole and in the blade of the leaf, are separated from 

 one another by plates of parenchymatous tissue, are placed 

 close together in the pulvinus forming an axial strand. 

 This axial strand is the part of the pulvinus possessing 

 greatest tensile strength, but the several layers of paren- 



* Sachs, J. von. tlber das Wachsthum der Haupt-und Nebenwurzeln. Arb. 

 d. Bot. Inst. Wiirzburg, Bd. I, p. 437, 1873, and Ges. Abhandl. , Bd. II, p. 826, 

 1893. Errera, L. Die grosse Wachsthumsperiode bei den Fruchttragern 

 von Phycomyces. Bot. Zeitung, Bd. 42, 1884. Wortmann, J. Zur Kennt- 

 niss der Reizbewegungen. Bot. Zeitung, Bd. 45, 1887. 



fNewcombe, F. C. Sachs' angebliche thigmotropische Kurven an 

 Wurzeln waren traumatisch. Beihefte z. Bot. Centralbl, xii., 1902. 



JSpauIding, V. M. The traumatropic curvature ol roots. Ann. of Bot., 

 viii., 1894. 



