^Se WATASE. [Vol. VI. 



differentiation with that of irritability, and to point out the 

 close parallels which exist between the two.^ 



" When by a touch on a trigger the explosion of a pistol is 

 caused, we do not say that the pistol is irritable, but when in 

 an organism a similar release of stored-up energy occurs, we 

 apply the term irritability to the phenomenon, and we call the 

 influence which produced the change a stimulus."^ Or, in the 

 words of Sachs, the stimulus is the name which is given to any 

 alteration in the environment of the irritable organs by means of 

 which stimulation is caused, such as intensity of light, variation 

 of temperature, alteration of electrical conditions, instantaneous 

 shocks, sudden pressure, etc. Considered from another point 

 of view, irritability is fundamentally nothing other than the re- 

 actions of the organs or organism towards the outside world, in 

 the way determined by the conditions of their inherited struct- 

 ure. Among such specific phenomena of irritability as heliotro- 

 pism, geotropism, etc., Sachs also includes such phenomena as 

 gall-formation, which is the abnormal reaction of the vegetable 

 protoplasm towards the abnormal stimulus caused by the insect. 

 He also includes in this category the phenomena that follow the 

 fecundation of the ovum, the ovum being an organism which, 

 under the stimulus from without of the fertilizing substance 

 reacts in so astonishing a manner that new processes of con- 

 figuration and growth arise from it. The association of these 

 two phenomena in the same category, viz. gall-formation, and 

 the development of the embryo following fecundation of the 

 ovum, is extremely suggestive. Darwin^ has already remarked 

 that every species of gall formed by different insects possesses 

 a specific anatomical structure and textural form " as if the 

 gall were an organism sui generis." The comparison between 

 the phenomena of gall-formation and development of the embryo 

 after fecundation of the ovum, becomes still more suggestive 

 when we remember that entirely different kinds of galls are 

 produced on the same plant by different insects, just as differ- 

 ent organisms are produced from the ova of the same animal 

 when the ova are fertilized by sperms of different parentage, 



1 I take the following accounts on irritability mostly from Sachs, Lectures on the 

 Physiology of Plants '; and also from Vines, Physiology of Plants. 



2 Francis Darwin, On Growth- curvatures in Plants, Nature, Vol. 44, 1891. 



3 Darwin, Animals attd Plants under Domestication, Vol. II. 



