ACTION OF LIGHT ON VEGETATION. 665 



light. The deeper the light penetrates, the more does it lose in intensity by ab- 

 sorption, reflexion, and dispersion. This loss however affects the different elements 

 of white light in very different degrees, as was shown by my investigations made 

 in 1859^ at present the only ones on this subject. The rays of greatest rc- 

 frangibility are in general almost entirely absorbed by the superficial layers of 

 tissue, while the red light penetrates most deeply. Of successive layers of an 

 apple, gourd, succulent stems, &c. only the outermost receives the light that falls 

 on it unchanged (independently of the reflexion from the surface) ; each deeper 

 layer is penetrated by light less intense than the preceding one, and of a different 

 composition. This change in the light which penetrates the tissue is principally 

 caused by colouring materials, especially chlorophyll, which have an absorptive 

 power for particular groups of rays, allowing others to pass through, and producing 

 in addition rays by fluorescence which v/ere not contained in the incident light. 

 But the relations of these changes of light in the tissues to the changes which 

 the light causes are not yet accurately known; not even in reference to chloro- 

 phyll, to which we shall again recur. What we have now said is intended only 

 to draw the attention of the student to the subject ; more exact investigations 

 must be made in working out the different questions which arise. 



B. Special, (i) Chemical Action of Light on Plants, (a) Formation of Chloro- 

 phylP. By the formation of the grains of chlorophyll the protoplasm becomes 

 differentiated into a colourless homogeneous part which forms the proper motile 

 or protoplasmic body of the cell, and into smaller distinct green portions which 

 remain imbedded in the former, the grains of chlorophyll. This process, as far as 

 concerns the differentiation, is independent of light, at least in flowering plants, 

 where the chlorophyll-grains are formed in the cells of the leaves even in the 

 dark. The chemical process, on the contrary, by which the green colour is pro- 

 duced has a complicated dependence on light. If, for instance, the temperature 

 is sufficiently high, the green colouring substance is formed in the cotyledons of 

 Conifers and in the leaves of Ferns in complete darkness as well as under the 

 influence of lights In Monocotyledons and Dicotyledons, on the contrary, the 

 grains of chlorophyll which are formed in the dark remain yellow, until they are 

 exposed to light even of small intensity, when they become green if only the 

 temperature is sufficiently high ; and the nearer, as I have shown, the temperature 

 approaches a definite maximum (25 to 30° C.) the quicker does the chlorophyll of 

 Angiosperms become green in the light. Provided therefore that the temperature is 

 favourable, the chlorophyll in the cotyledons of Conifers and the leaves of Ferns 

 does not require light in order to assume its green colour; while that in Angio- 



1 Sachs, Ueber die Durchleuchtung der Pflanzentheile ; Sitzungsber. der Wicn. Acad. iSrio. 

 Vol. 43 ; and Handb. der Exp.-Phys. p. 6. 



2 Sachs, Bot. Zeitg. 1862, p. 365, and Exp.-Phys. pp 10 and 318.— Sachs, Flora 1862, p. 21.^, 

 and 186+, no. 32 —Mohl, Bot. Zeitg. 1S61, p. 238.-Bohm, Sitzungsber. der \Vic»ncr Akad. vol. II. 

 Compare also Book I. sect. 6 of this work. 



■' P. Schmidt (Ueber einige Wirkungen des Lichts auf Pflanzen ; Dissertation, Breslau 1870, 

 p. 22) believes that these facts can be at least partially combated; but his experiments only prove 

 that the chlorophyll which is formed in the dark is again destroyed by long exposure to dark at a 

 high temperature (33"7 C), as is also the case with other plants. 



