ACTION OF LIGHT ON VEGETATION. 66? 



If we now turn to the observations on record, those of Wolkoff are the only 

 ones in which actual measurements have been made. With the assistance of 

 the photometric method contrived by Bunsen and Roscoe\ he showed first of 

 all that changes in the intensity of the highly refrangible light do not stand in any 

 appreciable relation to the exhalation of gas by water-plants. This is an ad- 

 ditional proof that these rays play only an extremely small part in this process, so 

 small indeed that in the experiments the actual effect might be concealed by other 

 causes (see p. 667). He next used as the source of light a dull glass plate 

 illuminated by daylight, at different distances from which he exposed the plants 

 (Ceratophyllum, Potamogeton, Ranunculus Jluitans) in a dark room ; and he ascer- 

 tained that the exhalation of gas was, within certain limits, nearly proportional to 

 the intensity of the light I There is probably however some particular intensity of 

 the efficient rays at which a maximum of gas is exhaled, and above which the 

 rapidity of the process again decreases and the plant suffers injury; but whether 

 this maximum intensity of light is attained or exceeded by the sunlight as it falls 

 on the surface of the earth cannot at present be determined. In reference to the 

 smallest degree of intensity of light at which exhalation of gas can still take place, 

 we have only the statement of Boussingault that a leaf of oleander ceased to ex- 

 hale oxygen after sunset^. 



The green colour of the chlorophyll of Monocotyledons and Dicotyledons 

 is not produced in the dark, as may be seen by enclosing plants in closely shut- 

 ting boxes of wood or metal, or in a dark cellar. The colouration begins however 

 when the amount of light is barely sufficient to read a book by ; and when it in- 

 creases to the ordinary brightness of a sunny summer day, the rapidity of the 

 change increases, and the colour becomes a deeper green than that produced 

 when plants are placed for a longer time in places not so strongly illuminated. 

 Famintzin nevertheless showed*, in the case of Lepidium sativum and Zea Mais, 

 that bleached seedlings become green more slowly in direct sunlight than in dif- 

 fused daylight. 



The small intensity of light which suffices for the formation of chlorophyll is 

 not sufficient for assimilation or for the formation of starch in the chlorophyll. Plants 

 (such as Dahlia, Faba, Phaseolus, Cucurbita, &c.), which rapidly become green in 

 the normal condition of full daylight, as well as in the diffused light of the back 

 of a room, still form no starch in their chlorophyll. They do however produce 

 chlorophyll in a window where, at the most, they enjoy but half the direct sun- 

 light and diffused daylight ; but, in harmony with this, the assimilation of these 

 plants is much less active in the window than in full daylight in the open air'\ 

 The following experiment gives a somewhat more precise result. Four plants of 

 TropcEolum majus grown from seed in the back of a room, all gave, when dried 

 at iio°C., a smaller weight than the seed; they had not assimilated, and died after 



Bunsen and Roscoe: Pogg. Ann. vol. loS. 



See also Pfefter: Arbeiten des botan. Inst, in Wiirzburg, Heft i, p. 41. 



Comp. rend. vol. 68, p. 410. 



Famintzin, Melanges biologiques ; Pelersbuig. vol. VI, p. 94, 1S66. 



Sachs, Bot. Zeit. 1862, No. 47; and 1864, p. 289 et seq. 



