74 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



None of these phrases give any explanation of the phenomena they are 

 used to connote, and hence when we say that an organ curves towards a source 

 of illumination because of its heliotropic irritability, we are simply expressing an 

 ascertained fact in a conveniently abbreviated form, without explaining why such 

 curvature is possible or how it is produced. Indeed, the classification of the 

 phenomena will differ according to whether predominant importance is attached 

 to the properties of the organism, the mode of response, the resulting growth, 

 or to the aims and purposes. Many observers have unfortunately devoted their 

 attention to artificially classifying the phenomena observed, and have entirely 

 neglected the explanation of the causes underlying them 1 . From a general 

 point of view we may distinguish between 



A. Essential formal conditions. 



(a) Sources of energy and food. 



(^) Stimulating factors (temperature, oxygen, &c.). 



B. Accessory non-essential factors. These act mainly as stimuli. 



With regard to the essential and non-essential stimuli and their results, we 

 may distinguish between 



A. Accelerating and retarding stimuli, which either influence the rapidity 



of growth generally, or locally, as in all curvatures. 



B. Formative or morphogenic stimuli, leading to local or general changes 



of shape. 

 (a) Exciting stimuli, which induce or inhibit the development of 



the organism, or of its resting primordia. 

 (b} Morphogenic stimuli, such as may lead to changes of shape or 



may even cause the primordium of a root to develop into 



a shoot. 

 (c) Regenerative and degenerative stimuli, which induce or suppress 



the formation of special organs. 



(d] Special generative stimuli, which induce formations not normally 

 produced. 



According to the mode of stimulation we may discuss in all cases 



A. Direct stimulation. 



B. Indirect stimulation, in which the disposition is altered and thus a 



special result produced. 



the organism, for in the vertical position the same stimulus is inoperative. In the case of 

 heliotropic curvatures, the intensity of the illumination differs considerably at the outset on the 

 two sides, even in semi-transparent unicellular organs, and we do not know whether it is the 

 direction of the light or the difference of illumination on the two sides which acts as a stimulus. 

 Only in the former case would there be any analogy with a geotropic curvature. Again, the term 

 ' trophic ' is used in Animal Physiology (trophic nerves, trophic fibres, trophism) in a widely 

 different sense to that suggested above. Further, the term ' auxesis ' has long been used in its original 

 general sense, ' heterauxesis ' merely denoting irregular growth whatever its origin (cf. Vines, 

 Physiology, 1886, pp. 376, 422), and this use has been generally accepted.] 

 1 Herbst, Biol. Centralbl., 1895, Bd. xv, p. 822. 



