ACTION OF LIGHT ON VEGETATION. 683 



as in the large green leaf.' He continues : — ' Leaves do not grow in the dark because 

 their cells cannot divide without the assistance of light;' while the exact converse is the 

 fact,— they do not divide because they do not grow. This error prevails throughout the 

 whole treatise, which in other respects contains a number of instructive observations. 



It must be observed in addition that the very small growth of leaves in the dark 

 is not a universal phenomenon even amongst Dicotyledons. The leaves produced 

 from the tuberous roots of the dahlia and beet grown in the dark, and even those 

 of Phaseolus attain very considerable dimensions, and sometimes, especially when the 

 temperature is high, almost the size of those developed in the lights 



Contri'vances for observing plants in light of deferent colours (or of different refrangi- 

 bility). In order to allow light of different degrees of refrangibility to act upon plants, 

 three methods may be adopted:— (i) The use of the spectrum; (2) The removal of 

 particular rays by absorbent media (glass or fluids); and (3) Coloured flames. 



(i) If a ray of light is decomposed by passing it through a prism, it is possible 

 to expose small plants or parts of plants to the action of narrow zones of the spec- 

 trum ; and hence to allow light of approximately equal refrangibility to act upon 

 them. Draper, Gardner^, Guillemin, and Pfeffer, have worked in this manner. In 

 using the spectrum it must however be observed that the intensity of the light in 

 its different parts is less than that of the light that passes through the slit in pro- 

 portion to the length of the spectrum. If the spectrum at the distance from the 

 prism where the observation is made is, for instance, 200 mm. long, but the slit only 

 1 mm. broad, the mean intensity of light of the whole spectrum is only 7200 of that 

 which passes through the even slit, if no light is otherwise lost, which is seldom the 

 case. Only a small luminous intensity must therefore be expected in the spectrum. 

 In order to obviate this difficulty, it is necessary that very intense light pass through 

 the slit, which may be effected by the use of condensing lenses. If, as is usually the 

 case, sunlight is employed, the ray to be decomposed must be kept in a fixed position 

 by a heliostat, or at least by a moveable mirror. 



(2) Absorbent tnedia. The defects which have been mentioned in observations 

 with the spectrum, as well as the considerable cost of a heliostat, are avoided when 

 coloured light is obtained by means of absorbent media. For this purpose discs of 

 coloured glass or strata of fluids enclosed between colourless glass plates may be 

 used. These last possess the advantage that almost any required amount of space 

 may be illuminated by the light in question, and that the transmitted light only 

 loses so much in intensity as is due to the small amount of absorption of the 

 transmitted rays by the coloured medium. It is a mistake, though a very common 

 one, to think that observations made with coloured screens are less exact than those 

 made with the spectrum; in general it is just the reverse; and which method should 

 have the preference must be decided in each case. 



The use of absorbent media is always subject to the disadvantage that they do 

 not generally transmit light of a single colour, but several different kinds of rays. 

 This disadvantage is especially the case with coloured glass plates; and, with the 

 exception of the deep red ruby and the very dark blue cobalt glass, there are scarcely 

 any kinds which answer our purpose. It is more practicable to obtain coloured fluids 

 of the desired quality, although here also the number that can be used is small. 

 The two which have been already mentioned are particularly useful, inz. a saturated 

 solution of potassium bichromate, and a dark solution of ammoniacal copper oxide ; by 

 means of these, with the right concentration and thickness of the stratum, experi- 

 ments can be contrived so as to split white daylight exactly into two halves, the 

 first solution transmitting the less refrangible rays from the red to the green, the 



^ See infra. Sect. 20. 



2 Gardner, Froriep's Notizen 1S44, vol. 30, No. 1 1. —Guillemin, Ann. des Sci. Nat. 1S57, vol. 

 VII, p. I Co. 



