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 if 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 Paper*, 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 

 them support and protection. The tendril of 

 the vine is internally similar to that of the 

 ampelopsis, though its external form and mode 

 of attaching itself, by twining round any slen- 

 der body, are very different. Some young 

 plants of this species which had been raised 

 in pots in the preceding year, and had been 

 headed down to a single bud, were placed in 

 a forcing house, and the shoots from these 

 were bound to slender bars of wood, and train- 

 ed perpendicularly upwards. Their tendrils, 

 like those of the ampelopsis, when first emit- 

 ted, pointed upwards, but they gradually form- 

 ed an increasing angle with the stems, and 

 ultimately pointed perpendicularly downwards, 

 no object having presented itself to which they 

 could attach themselves. Other plants of the 

 vine, under similar circumstances, were trained 

 horizontally, when their tendrils gradually de- 

 scended beneath their stems, with which they 

 ultimately stood very nearly at right angles. 

 A third set of plants were trained almost per- 

 pendicularly downwards, but with an inclina- 

 tion of a few degrees towards the north, and 

 the tendrils of these permanently retained very 

 nearly their first position relatively to their 



stems; whence it appears that these organs, 

 like the tendrils of the ampelopsis and the 

 claws of the ivy, are to a great extent under 

 the control of light. A few other plants of the 

 same species were trained in each of the pre- 

 ceding methods, but proper objects were placed 

 in different situations near them, with which 

 their tendrils might come into contact, and I was 

 by these means afforded an opportunity of ob- 

 serving with accuracy the difference between 

 the motions of these and those of the ampelop- 

 sis under similar circumstances. The latter al- 

 most immediately receded from light, by what- 

 ever means that were made to operate upon 

 them; and they did not subsequently show any 

 disposition to approach the points from which 

 they once receded. The tendrils of the vine, 

 on the contrary, varied their positions in every 

 period of the day, and afterwards returned 

 again during the night to the situations they 

 had occupied in the preceding morning, and 

 they did not so immediately or so regularly 

 bend towards the shade of contiguous objects. 

 But as the tendrils of this plant, like those of 

 the ampelopsis, spring alternately from each 

 side of the stem, and as one point only in three 

 is without a tendril, and as each tendril sepa- 

 rates into two divisions, they do not often fail 

 to come into contact within their reach, and 

 the effects of contact upon the tendrils are al- 

 most immediately visible. It is made to bend 

 towards the body it touches, and, if that body 

 be slender, to attach itself firmly by twining 

 round it, in obedience to certain causes. The 

 tendril of the vine, in its internal organization, 

 is apparently similar to the young succulent 

 shoot and leaf-stalk of the same plant ; and it 

 is abundantly provided with vessels or pas- 

 sages for the sap ; and I have proved that 

 it is alike capable of feeding a succulent 

 shoot, or a leaf when grafted upon it. It ap- 

 pears, therefore, I conceive, not improbable 

 that a considerable quantity of the moving 

 fluid of the plant passes through its tendrils, 

 and that there is a close connection between, 

 its vascular structure and its motions." (Ibid. 

 p. 166. 



"The stems of the potato," he adds in an- 

 other place, "as of other plants, rise perpen- 

 dicularly under the influence of their unerring 

 guide, gravitation, so long as they continue to 

 be concealed beneath the soil; but as soon as 

 they rise above it they are, to a considerable 

 extent, under the control of another agent, 

 light. Each inclines in whatever direction it 

 receives the greatest quantity of it, and conse- 

 quently each avoids and appears to shun the 

 shade of every contiguous plant; gravitation 

 labouring to give a perpendicular, the other a 

 horizontal direction to the leaves, and the com- 

 parative power of one agent increasing, as that 

 of the other decreases." {Ibid. p. 300306.) 



The opinions of Liebig, as to the chemical 

 changes produced in plants, by the action of 

 light, and its withdrawal, seem confirmed, in 

 some degree, by those of Davy. " In the 

 changes that take place in the composition of 

 the organized parts," said that excellent phi- 

 losopher, "it is probable that saccharine com- 

 pounds are principally formed during the ab- 

 sence of light; gum, woody fibre, oils, and 



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