112 PLANT PHYSIOLOGY 



the experience of animal physiologists. This late intimate contact 

 of so nearly related disciplines, which earlier had been led up to, 

 and again and again failed of, has shown itself in the highest degree 

 fruitful ; and the physiology of irritability, which earlier was pushed 

 forward by Sachs, and later by Pfeffer and his school, and is at the 

 present time in the forefront of interest, is referable to the beneficial 

 influence upon plant by animal physiology. I beg to be allowed to 

 emphasize this influence still more, and to indicate the interaction 

 of these two sister sciences. 



In spite of Fechner's earlier suggestion on the point, there is no- 

 thing to be found in Sachs' Experimental Physiologic on the mat- 

 ter of irritability. The most important and most frequently adduced 

 phenomena, as heliotropism and geotropism, were referred to tissue 

 tensions and similar purely mechanical effects. Almost all plant 

 physiologists followed the path which was pointed out by the gifted 

 morphologist, Hofmeister. Only a few of the most striking move- 

 ments of plant organs, as, e. g., those of the leaves of Mimosa pudica, 

 were spoken of by Sachs as "the so-called phenomena of irritability." 

 He stood in this under the influence of the great animal physiologist, 

 Bruecke, who, in order to get a more comprehensive idea of the life 

 of organisms, took up plant physiology and studied closely the 

 sensitiveness of Mimosa pudica. 



In his Pflanzenphysiologie, still following Hofmeister, Sachs ex- 

 plains positive geotropism as a bending under weight, that the 

 root-tip as a result of its weight when the root is placed in a hori- 

 zontal position, bends downward in the subapical part, which is 

 composed of soft, plastic, and tender cells. Sachs says expressly 

 that, just as the end of a piece of sealing-wax bends downwards 

 when the part behind is softened by heat, the heavy and stiff end 

 of the root bends out of the inclined, or horizontal, into the vertical 

 position. 



In his later writings this account, which was out of all harmony 

 with the facts of anatomy, was not held to, for the conception of the 

 phenomenon of irritability underwent a total change, doubtless 

 under the undoubted influence of animal upon plant physiology. 

 The way we to-day regard irritability in plants is a reflection of the 

 matter from the animal point of view. Pfeffer took this position in 

 the first edition of his celebrated Plant Physiology, and yet more 

 clearly and decidedly in the second edition, recently completed. 

 Nowadays the phenomena of heliotropism and geotropism not to 

 pass beyond the examples cited are regarded as those of irritabil- 

 ity in the sense of the animal physiologists. The causes of stimulation 

 (gravitation, light, etc.) have been determined, the point of recep- 

 tion shown, transmission of stimuli proved, and the whole course 

 determined in detail. The value which accrued to plant physiology 



