836 MECHANICS OF GROWTH. 



The increments of growth of primary roots of seedlings of Vic'ui Faba were aft 

 follows: — 



In the dark. In diffuse light. 



In 5 roots as 309 to 272 



II 743 612 



9 612 416 



In these cases a tendency of the roots was observed, though not a very decided one, 

 to positive heliotropic curvature. The difference in the rapidity of growth would no 

 doubt have been greater if the increments in the same time had been compared during 

 the day only. 



The long narrow leaves of many Monocotyledons exhibit the same phenomena as 

 internodes and roots, becoming considerably longer in permanent darkness than under 

 normal conditions, and showing positive heliotropic curvature when the light from the 

 two sides is unequal. The plane of curvature may coincide with the plane of the leaf, 

 so that one margin may be considerably longer than the other, and the whole leaf there- 

 fore unsymmetrical. I have observed this very evidently in a plant of Friiillarla impe- 

 rialis grown in a window ; those leaves only which sprang exactly from the side of the 

 stem exposed to light being symmetrical like those growing in the open air. We have 

 at present no observations on the daily periodicity in these leaves caused by light. 



Observation of the broad netted-veined leaves of Dicotyledons is much more difficult. 

 From the fact that in the dark they remain smaller, and often very much so, than 

 under normal conditions, it might be concluded that their superficial growth presents 

 exactly opposite phenomena to those of internodes and the long leaves of Monocoty- 

 ledons. But Batalin has shown that it is sufficient to expose etiolated plants now and 

 then to light — the time not being long enough for them to become green — for their 

 growth in the dark to be afterwards considerably promoted. This leads to the suppo- 

 sition that light causes in etiolated leaves a chemical change which is not connected with 

 assimilation, by which they are enabled to grow further in the dark. In any case this 

 phenomenon shows that there is no real contradiction between the growth of these 

 leaves and that of internodes, and that the reason why they become larger under the 

 normal conditions of light than in permanent darkness is not because light has a directly 

 favourable influence on the growth of the cells of these leaves. The recent experi- 

 ments of Prantl ^ rather favour the hypothesis that green — and therefore healthy and 

 normal— leaves exhibit the same diurnal periodicity of growth as positively heliotropic 

 internodes. He succeeded, by a number of measurements both in breadth and length 

 of the leaves of Cucurbita Pepo and Nicotiana labacum, taken at intervals of three 

 hours, in constructing curves of growth, which, in spite of adverse fluctuations of temper- 

 ature, rose from evening to morning, attained a maximum after sunrise, and then fell 

 during the day till evening ; exactly what I showed to be the case with positively helio- 

 tropic internodes. If this general law is established, it results that the broad netted- 

 veined leaves of Dicotyledons grow more quickly in the dark than in the light, and are 

 therefore hindered in their growth by light. But when such leaves remain nevertheless 

 smaller in permanent darkness because they cease growing earlier, this must be inter- 

 preted as an unhealthy condition depending on the suspension of certain processes of 

 metastasis which must precede growth and which are induced by light. In conformity 

 with this hypothesis we must suppose that in leaves which unfold under the alternate 

 influence of day and night, growth is directly hindered by light ; but that at the same 



^ Arbeit, des bot. Inst, in Wlirzburg, Vol. I, p 382. [See also Stebler, Ueb, Blattwachsthum, 

 Jahrb. f. wiss. Bot. XI, 1877. From his observations Stebler draws the erroneous conclusion that 

 light promotes the growth of leaves (of Monocotyledons). For a criticism and further observations 

 see Vines, The Influence of Light upon the Growth of Leaves, Arb. d. bot. Inst, in Wiirzburg, II, 



1878.] 



