J^^l MECHANICAL LAWS OF GROWTH, 



It appears therefore that the growing part of a plant must be subjected for a con- 

 siderable time to any particular temperature in order that its growth may attain the 

 greatest rapidity corresponding to this temperature. 



Koppen's results are only in apparent contradiction to my own, according to which 

 the curve of growth rises and falls with that of temperature; for it is possible that the 

 entire growth in a given time may be greater when the temperature remains at a con- 

 stant elevation than when it oscillates above and below it. 



Sect. 20.— Action of Light on Growth. — Heliotropism \ Since we shall 

 now pay exclusive attention to the questions whether and in what way light pro- 

 motes or retards quantitatively the superficial growth of the cell-wall, w^e may for 

 the time leave entirely out of consideration those cases where it changes or may 

 possibly change qualitatively the physiological and morphological nature of the newly 

 formed organs. 



The dependence of growth on light has already been spoken of in general terms 

 in Sect. 8 ; and it was there especially insisted on that, in order to avoid serious 

 misconceptions, this must be distinctly separated from the question of the part 

 taken by light in assimilation. Here also we are concerned only with the processes 

 of growth itself, since we always start from the point at which the cells or organs 

 concerned have already obtained a sufficient quantity, or even excess, of formative 

 materials. 



It has been already stated that the various parts of the flower grow as readily 

 in permanent darkness as in light. Most internodes, on the contrary, as has been 

 explained in Sect. 18, grow more slowly when exposed to light on all sides, and 

 remain shorter than when growing in the dark ; when the light reaches them from 

 one side only, they curve concavely towards the source of light. Other organs how^- 

 ever, as root-hairs, tendrils, and some internodes, become longer on the side exposed 

 to light than on that left in the dark. We have seen also that the leaves of Ferns 

 and Dicotyledons soon cease growing in the dark and remain small. These observ- 

 ations show clearly enough that different cells and organs are differently affected 

 by fight as respects their growth. Since the light itself remains the same and there 

 is a supply of formative materials, any explanation of these differences must aim 

 at showing how the inherited organisation of the plant must have been altered just 

 in this way and no otherwise by the oscillations of the ether. It is however at pre- 

 sent quite impossible to give such an explanation^, since far too little is yet known 



' A. P. De Candolle, Physiologic vegetale, Paris 1832, vol. Ill, p. 1079. — Sachs, Bot. Zeitg. 

 1863, Supplement, and 1865, p. 117. — Ditto, Experimental-Physiologic, Sect. 15. — Hofmeister, 

 Lehre von der Pflanzenzellc, Sect. 36. — Kraus, Jahrb. fiir wissensch. Bot. vol. VII, p. 209 et $eq. — 

 Batalin, Bot. Zeitg. 1871, No. 40. 



^ If Miiller, in the second part of his Botanischc Untcrsuchungen (Heidelberg 1872), gives the 

 impression of having achieved this with but little difficulty, this only shows how far he is from a 

 true method of investigation. 



