LIGHT. 



LIGHT. 



The action of light upon the growing plant 

 is in every point of view full of interest to the 

 cultivator: "If all the branches of a tree, ex- 

 clusive of one," said Mr. T. M. Knight (and he 

 was one of the ablest of modern vegetable 

 physiologists), "be much shaded by contigu- 

 ous trees, or other objects, the branch which is 

 exposed to the light attracts to itself a large 

 portion of the ascending sap, which it employs 

 in the formation of leaves and vigorous an- 

 nual shoots, whilst the shaded branches be- 

 come languid and unhealthy. The motion of 

 the ascending current of sap appears, there- 

 fore, to be regulated by the ability to employ it 

 in the trunk and branches of the tree , and 

 this current passes up through the alburnum, 

 from which substance the buds and leaves 

 spring. But the sap which gives existence to, 

 and feeds the root, descends through the bark, 

 and it' the operation of light give ability to the 

 exposed branch to attract and employ the as- 

 cending or alburnous current of sap, it ap- 

 pears not improbable that the operation of 

 proper food and moisture in the soil, upon the 

 bark of the root, may give ability to that organ 

 to attract and employ the descending or cor- 

 tical current of sap." (Selection of Papers, p. 

 160.) "M. Decandolle, I believe, first ob- 

 served that the succulent shoots of trees and 

 herbaceous plants, which do not depend upon 

 others for support, are bent towards the point 

 from which they receive light, by the contrac- 

 tion of the cellular substance of their bark 

 upon that side, and I believe his opinion to be 

 perfectly well founded. The operation of light 

 upon the tendrils and stems of the Ampelopsis 

 and ivy appears to produce diametrically op- 

 posite effects, and to occasion an extension of 

 the cellular bark wherever that is exposed to its 

 influence ; and this circumstance affords, I think, 

 a satisfactory explanation why these plants ap- 

 pear to seek and approach contiguous opaque 

 objects, just as they would do if they were 

 conscious of their own feebleness, and of power 

 in the objects to which they approach, to afford 

 tlu-in support and protection. 



Influence of the different rays of the solar spec- 

 trum upon vegetation. 



The process of germination is essentially a 

 chemical one. The seed is placed in the soil, 

 supplied with a due quantity of moisture, and 

 maintained at a certain temperature which 

 must be above that at which water freezes. 

 Air must have free access to the seed, which, 

 it' placed so deep in the soil as to prevent the 

 permeation of the atmosphere, nevev germi- 

 nates. After this progresses beyond the first 

 stages, and leaves are formed, the plant com- 

 mences to absorb carbonic acid from the atmo- 

 sphere through the under surface of the leaves 

 and the whole of the bark. It at the same 

 time derives an additional portion from the 

 moisture which is taken up by the roots. 



The phenomena of vegetable life, such as 

 germination, growth, efflorescence, the decom- 

 position of carbonic acid gas, the secretions 

 of the acid and alkaline juices, oils, wax, resin, 

 &c., are all results of solar influences, exerted 

 by the separate agencies residing in the differ- 

 ent coloured bands of light into which the white 

 light of the sunbeam is resolved by means of 



j the prism. These variously coloured rays have 

 not the same illuminating power, nor do they 

 possess the same heat-giving property. The 

 yellow rays give the most light; the red rays 

 j have the function of heat in the highest de- 

 I gree. In addition to these rays the sunbeam 

 possesses another, with the power of pro- 

 ducing chemical change, or actinism. A yellow 

 glass allows light to pass through it most 

 freely, but it obstructs actinism or chemical 

 agency. A deep-blue glass, on the contrary, 

 obstructs the passage of light, but it offers no 

 interruption to the actinic, or chemical rays. 

 A red glass cuts off most of the rays, except 

 those which have peculiarly a calorific, or heat- 

 giving power. 



It has been found by experiment, that if, 

 above the soil in which seed is placed, a pure 

 yellow glass is fixed, the chemical change which 

 marks germination is prevented; if, on the 

 contrary, a blue glass be employed, it ig accel- 

 erated. Seeds placed beneath the soil and 

 covered with a cobalt-blue finger-glass have 

 been found to germinate many days sooner 

 than such as were at the same time exposed 

 to the ordinary influences of sunshine prov- 

 ing the necessity of the agency of actinism to 

 this first stage of vegetable life. Plants, how- 

 ever, grown under such media present much 

 the same conditions as those reared in the 

 dark are succulent instead of woody, and 

 have yellow leaves and white stalks. The de- 

 velopment of the leaf is prevented whilst that 

 of the stalk is increased. This shows that the 

 chemical principle alone of the sun's rays is 

 not sufficient for the proper development of the 

 various functions of vegetation. When, how- 

 ever, the plant is placed under the full influ- 

 ence of light, as separated from actinism, by 

 the action of yellow media, wood will be 

 formed abundantly, the plant grow most 

 healthfully, and the leaves assume that dark- 

 green which belongs to tropical climes. Un- 

 der the influence of isolated light it is found 

 that plants will not flower. When, however, 

 the subject of experiment is brought under 

 the influence of a red glass, the whole process 

 of floriatiou and the perfection of seed is ac- 

 complished. 



In spring, when the process of germination 

 is most active, the chemical rays are most 

 abundant in the sunbeam. As the summer 

 advances, light, relatively to the other forces, 

 is largely increased, and at this season the 

 trees of the forest and all cultivated plants are 

 engaged in making wood. Still later in the 

 season the heating rays predominate, exert- 

 ing their agency in maturing the seeds and 

 fruits. 



Thus under the influence of the sunbeam, 

 vegetable life is awakened, continued, and 

 completed ; a wondrous alchemy is effected ; 

 the change in the condition of the solar radia- 

 tions determines the varying conditions of 

 vegetable vitality ; and in its progress those 

 transmutations occur, which at once give 

 beauty to the exterior world, and provide fo* 

 the animal races the necessary food by which 

 their existence is maintained. (Professor 

 Hunt.} 



The influences exerted upon growing plants 



