ACTION OF LIGHT ON GROWTH. J ^^ 



these internodes. In the case also of positively heliotropic roots (as those of Zm Mais 

 Lemna, Gucurbita, Pistia, &c.), it may be supposed that if exposed to daylight they 

 would exhibit the same alternation as internodes ; but this is not yet fully established. 

 Wolkoff has, on the other hand, already shown in the case of some roots that when 

 they develope in water behind a transparent glass plate they grow more quickly in per- 

 manent darkness than under the alternation of day and night. Twelve primary roots 

 of seedlings of Pisum snti-vum gave, for example, the following results:— 

 Day. ' Successive increments. 



In the dark. In (Uffusetl litcht. 



ist 195 mm. 161 mm. 



2nd 239 153 



3rd 250 210 



4th 126 113 



5th 113 78 



In the 5 days. 923 mm. 715 mm. 



The increments of growth of primary roots of seedlings of Ficia Fnba were as 

 follows : — 



In the dark. In diffused light. 



In 5 roots as 309 to 272 



II 743 ^'2 



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 \i 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 Fritillaria impe- 

 rialis grown in a window ; those leaves only which sprung 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 change which does not consist in chemical 

 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 PrantP 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 



^ Compare also Sachs, Arbeit, des hot. Inst, in Wiirzburg, Heft II, p. il 



3 C 2 



