ACTION OF LIGHT ON VEGETATION. 74 1 



additional 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. 744). He next used as the source of light a dull glass plate 

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

 {Ceratophylluin^ Potamogeton, Ranunculus fluiians) 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 ^ 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 exhale 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 shutting 

 '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- 

 granules. Plants (such as Dahlia, Faha, 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-granules. They 

 do however produce starch when placed in a window where, at the most, they enjoy 

 but half the direct sunlight and diffused daylight ; but, in harmony with this, the 

 assimilation of these plants is much less active . in the window than in full daylight m 

 the open air*. The following experiment gives a somewhat more precise result. 

 Four plants of Tropceolum viajus 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 consuming the reserve-material, although in the shade of the room 

 they all produced green leaves. Four other plants of the same species which 

 germinated at the same time grew for three months, exposed for only seven hours 

 each day to the diffused light of a west window in the forenoon ; they formed 

 nearly 5 grammes of dry substance. Four other plants which were exposed in a west 

 window from i p.m. till the following morning, and therefore to the afternoon sun- 

 shine, produced also only 5 grammes ; while four other plants which stood in the 



' See also Pfeffer, Arbeiten des botan. Inst, in Wiirzburg, Heft I. p. 41. 



^ Comp. rend. vol. 68. p. 410. 



' F'amintzin, Melanges biologiques; Petersbourg, vol. VI. p. 94, 1866. 



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



