6^2 



produce leaves which at first unfold as green leaves and later, when they 

 become old, become light yellow in places. In the same way, many yellow 

 garden varieties of woody plants only become a bright yellow with strong 

 insolation ; in the shade they remain green. 



Ewart^ observed in tropical plants a complete bleaching of the chloro- 

 phyll grains as a result of an excess of light. If the light stimulus increases 

 above the specific optimum, the optimal and maximal development of gases 

 at first continues for a short time, but then follows a condition of exhaus- 

 tion^. If this excessive stimulation does not last too long, the plant can 

 recover its normal activity. This over-stimulation can also occur under 

 our normal light conditions, if the plant, by nature, belongs among shade 

 plants. Weiss^ cites a fine example of this in Poly podium vulgar e, a de- 

 cided shade plant, as contrasted with Oenothera biennis which is distinctly 

 a sun plant. With a favorable temperature, the latter produced about 

 three times as much carbon dioxid in direct sunlight as in diffuse light ; 

 while the former assimilated more energetically in dillfuse light. Diffuse 

 daylight can, in fact, act to arrest the growth of roots which are accustomed 

 to the dark, as Kny found in lupines, cow beans, and water cress*. In this, 

 he observed in lupines usually a decrease of growth in thickness and a 

 retarding of the development of the central cylinder, if the growth in length 

 increased. 



The works of Dixon, Dixon and Wigham, Joseph and Prowazek, Max 

 Koernicke and Hans Molisch'^ prove a very decided arrestment of growth 

 from the use of Rontgen and radium rays. 



An abnormal thickening and a wrinkled surface were observed in pea 

 roots, which could be traced, apparently, to differences in internal tension. 

 Contractions are produced by the increase in the radial diameter of the cells 

 of the inner bark parenchyma, together with a shortening of the longi- 

 tudinal diameter. It was found in other experiments with vetches and horse 

 beans that the roots turned brown and their growth was arrested. But 

 after 8 to lo days they grew further, after having thrown of¥ the outermost 

 tips in the form of brown caps, and formed new root tips directly behind 

 these. Normal lateral roots were produced immediately. The arrest of 

 growth 'is less in organs containing chlorophyll. In seedlings a cessation in 

 the growth in length has been observed but no dying back. The leaves 

 became somewhat smaller than in normal specimens. Dixon" could not 



1 Ewart, A. J., The effects of tropical insolation; cit. Just's Jahresber. 1899, 

 I. p. 87. 



- Pantanelli, Enrico, Abhangiffkeit der Sauerstoffau.sscheidiins- belichtetei- 

 Pflanzen von iiusseren Faktoren. Jahrb. f. wiss. Bot. 1903, Vol. XXXIV, p. 167. 



3 Weiss, Fr., Sur le rapport entre I'intensit^ lumineuse et I'^nergie assimilatrice 

 chez les plantes appartenant a. des types biologiques differents. Compt. rend. Paris 

 CXXXVII, 1903, p. 801. 



■* Knv, Ij., liber den Einfluss des Lichtes auf das "Wachstum der Bodenwurzeln. 

 Jahrb. f. wiss. Bot. 1902. Vol. 28, p. 421. 



5 Seckt, Hans. Die Wirkung- der Rontgen- und Radiumstrahlen auf die Pflanzp. 

 Sammelreferat. Naturwiss. "Wochenschrift; 1906, No. 24. 



6 Dixon, Henry, Radium and plants. Nature, London LXIX; cit. Just's Bot, 

 Jahresber. 1903, II, p. 567. 



