ACTION OF LIGHT ON VEGETATION. 743 



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 were 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 Planls. {a) Formation 0/ Chloro- 

 phyll\ In the formation of the chlorophyll-granules the protoplasm becomes 

 differentiated into a colourless continuous part which forms the proper motile pro- 

 toplasmic body of the cell, and into smaller distinct green portions which remain 

 imbedded in the former, the chlorophyll-granules. This process, as far as concerns 

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

 chlorophyll-granules are formed in the cells of the leaves even in the dark. The 

 chemical process, on the contrary, by which the green colour is produced 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 light 2. 

 In Monocotyledons and Dicotyledons, on the contrary, the chlorophyll-granules 

 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 sufficiendy 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 Angiosperms does require it ; and in both 

 cases the change does not take place at a low temperature (see p. 729). It may be 

 added here that the subterranean protonema of Mosses contains chlorophyll, though 

 but in small quantity. 



It may be concluded from such observations as have been made that all the 

 visible parts of the solar spectrum have the power of turning the etiolated chlorophyll- 



^ Sachs, Bot, Zeitg. 1862, p. 365, and Exp.-Phys. pp. 10 and 318. — Sachs, Flora, 1862, p. 213, 

 and 1864, no. 32,— Mohl, Bot. Zeitg. 1861, p. 238. — Bohm, Sitzungsber. der Wiener Akad. vol. TI. 

 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 (337° C), as is also the case with other plants. 



^ [Elfving has. found (Arb. d. bot. Inst, in Wiirzburg, II. 3, 1880) that exposure to light at a 

 temperature which is not sufficiently high to produce chlorophyll leads to an increased formation of 

 the yellow colouring matter (etiolin) in etiolated seedlings. 



There is reason to believe that chlorophyll is derived from etiolin (see Wiesner, Eiitstehung des 

 Chlorophylls, Wien, 1877) ] 



