EFFECT OF EXTEENAL AGENCIES ON GROWTH. 605 



Effect of Light. The effect of light on chlorophyll and the deoxida- 

 tion which takes place when that substance is exposed to its agency have 

 been already mentioned. But exposure to light is only required for a 

 certain time to allow of the storing up of reserve materials. This latter 

 can be utilized and growth take place in the absence of light. White 

 light is composed of various rays differing, not only in colour, but also in 

 their influence on plants. Most observers, as Daubeny, Hunt, Draper, and 

 Sachs, have considered the luminous (yellow) rays of low refrangibility to 

 be the most potent in affecting the decomposition of carbonic dioxide ; but 

 Timiriazeff and Miiller assert, from recent experiments with the spectro- 

 scope, that the maximum of assimilation coincides with the maximum of 

 heat, not with that of luminous intensity. Carbonic dioxide is decomposed by 

 the rays which are absorbed by the chlorophyll, and of these the most 

 potent are those which have the greatest heating power. This conclusion 

 needs further corroboration before it can displace the generally accepted 

 view. 



The highly refrangible rays, blue, violet, &c., " influence the rapidity of 

 growth, alter the movements of the protoplasm, compel swarm-spores to 

 adopt a definite direction to their motion, and change the tension of the 

 tissues of the motile organs " (Sachs, Baranetzky, Pfeffer, Prillieux). 

 The degree of effect produced by light naturally varies, not only with its 

 quality, but also with its intensity. Diffused fight, which may be suffi- 

 cient for the production of chlorophyll, is powerless to induce the further 

 changes which result in the formation of starch. In the case of those 

 seeds (Convolvulus, Conifers) in which the embryo, though still enclosed 

 within the opaque coverings of the seed, is found to be green, the chlo- 

 rophyll must be formed independently of light from the reserve substances 

 in the perisperm. Differences of this character may also be produced by 

 the varying degree to which light penetrates different tissues, and the 

 different modifications to which it is subjected in its passage through 

 tissues of different densities. 



Effect of Light, or of its absence, on Growth. Cell-division may take 

 place either in darkness or in light, provided the requisite materials be at 

 Land. In Algae, like Spirogyra, which have little reserve material assi- 

 milation goes on in the daytime, cell-division at night. Hence, as 

 Sachs remarks, in the higher plants assimilation and growth may go on at 

 the same time in different parts of the same organism, while in the 

 simpler organism, where the same cell has to perform both functions, it 

 assimilates by day and divides by night. When a growing shoot is grown 

 in darkness, as in the case of potatoes sprouting in a dark cellar, not only 

 is a blanched condition observable, but also a great elongation. This is 

 also seen in badly lighted hot-houses, or in dark corners of gardens, thick 

 shrubberies, &c., where the plants become " drawn," as the gardeners 

 say. When such shoots are exposed to light the extension of the shoot 

 is arrested and growth in length is retarded, and, as Sachs has shown, a 

 periodical oscillation in the rapidity of growth is caused by the alterna-. 

 tion of day and night, when the temperature is nearly constant. The 

 growing internode exhibits a maximum of growth about sunrise, which 

 decreases till about midday, when it reaches its minimum. Contrary to 

 what happens in the stem, the leaves are arrested in their growth in 

 obscurity. Etiolated leaves cease to grow at the point where, under 



