GROWTH 201 



growth, heat on the contrary accelerates it ; which as a 

 matter of fact was known very long ago to gardeners, 

 who base upon it their method of forcing plants, ac- 

 celerating or retarding their growth in order to make 

 them develop in readiness for a certain date. Evidently 

 the question as to the time when the plant grows most 

 is not as simple as it seemed to be. It is dark in the 

 night but also colder ; it is light in the day-time but 

 also warmer. It is difficult to say beforehand which 

 of the two influences will preponderate in any case ; 

 the only obvious thing is that growth must be most 

 energetic during a dark and warm night, and least 

 energetic on a bright and cold day. 



We have brought forward the simplest explanation 

 of the phenomena of heliotropism, i.e. of the inclination 

 of stems towards the source of light ; but many people 

 are not satisfied with it, because together with the 

 general phenomenon of bending towards light, there are 

 also cases, comparatively rare it is true, of bending 

 away from light ; or, as it is sometimes stated, amid 

 the preponderating phenomena of positive heliotropism 

 there are some rare cases of negative heliotropism. This 

 objection, which has obliged many botanists to give up 

 the explanation just brought forward, may be easily re- 

 moved on the strength of the more recent discovery that 

 one-sided heating can cause phenomena similar to those 

 of heliotropism, and called thermotropic phenomena. 

 Evidently the result of thermotropism will be quite 

 the opposite. Heat accelerates growth — therefore the 

 heated part will grow more quickly and the organ will 

 bend away from the source of heat. But sunshine acts 

 at the same time both as light and as heat ; it is obvious, 

 therefore, that according as the one or the other influence 

 predominates the organ will bend either towards or 

 away from the light. We have just mentioned that 

 the production of different effects attributed to a single 

 factor may depend either upon a difference in the pro- 



