ACTION OF LIGHT ON LEAVES.

Wairarapa Daily Times, Volume 5, Issue 1340, 30 March 1883, Page 2

ACTION OF LIGHT ON LEAVES.

; i (From. Agricultural Gazette.) Leaves possess blm-powerof turning to the light, now called heliotropism," and especially of so placing their upper surface as to form a right.tingle "ip the direction of the light, lit'has "been surmised that the horizontal position of leaves, and especially tho position with regard to. the direction of light, was due to tho conjoint action of/gravitation or " geotropisni," of " heliofcropisin," and of the greater relative force of growth on one or the other surface, the particular direction assumed by the leaves being supposed to be due to the balance between these forces; but by means of experiments made .with a view of annulling or counteracting the., effects of gravitation and of unequal growth, Mr Francis Darwin has.shown that the power which leaves have!of placing themselves at right anglesto incident light is due to a special 'sensitiveness, which is capable of regulating the action of other forces, whether external to the plant, as. that of gravitation, or internal, such as that regulating the direction of growth. The movements iof 1 the leaves, towards the 1 light are different from . others which i are of a periodic character, in that they are influenced by the direction rather than the intensity of the light. Growth, • therefore, is carried on independently of and not contemporaneously with nutrition by the leaf, so': far: as the latter consists in the decomposition of carbonic acid and the fixation of the carbon. Thus.it.'has been shown by Dr. Vines that leaves will grow—l in darkness, or under the .influence of blue.light; 2, in air deprived. of carbonic acid; and 3, even in the absence of chlorophyll. But although there is thus shown to. be no direct relation between nutrition and growth, yet there'is, ofcourse, an indirect relation; growth under the. apparently adverse conditions'just mentioned being only possible incases where there is available some store of nourishment previously formed by assimilation,