loo FUNCTIONS OF THE GREEN TWIGS. 



days appeared blighted, while on the patch previously exposed to the 

 red, some additional plants sprung up.''* 



Besides the rays of heat and of light, the sun-beam contains what 

 have been called chemical rays, not distinguishable by our senses, but 

 capable of being recognized by the chemical effects they produce. 

 These rays appear to differ in kind, as the rays of different coloured 

 light do. It is to the action of these chemical rays on the leaf, and 

 especially to those which are associated with the blue light in the solar 

 beam, that the chemical influence of the sun on the functions of the leaf 

 is principally to be ascribed. 



It cannot be doubted that the warmth and moisture of a tropical cli- 

 mate act as powerful stimulants — assistants it may be — to the leaH in 

 the absorption of carbonic acid from the air, and in that rapid appropria- 

 tion (assimilation) of its carbon by which the growth of the plant is has- 

 tened and promoted. But the bright sun, and especially the chemical in- 

 fluence of his beams, must be regarded as the main agent in the wonderful 

 development of a tropical vegetation. Under this influence the growth 

 by the leaves at the expense of the air must be materially increased, 

 and the plant be rendered less dependent upon tiie root and the soil for 

 the food on which it lives, f 



V. Tlie rapidity with which a plant grows has an important influence 

 upon the share which the hark is permitted to take in the general 

 nourishment of the whole. The green shoot performs in some degree 

 the functions of the leaf. In vascular plants, therefore, which in a con- 

 genial climate may almost be seen to grow, the entire rind of a tall tree 

 may more or less effectually absorb carbonic acid from the atmosphere, 

 during the presence of the sun. The broad leaves of the palm tree, 

 when fully developed, render the plant in a great degree independent of 

 the soil for organic food — and the large amount of absorbing surface in 

 the long green tender stalks of the grasses, and of their tropical ana- 

 logues, must malerially contribute to the same end. Hence the pro- 

 portion of organic matter derived from the air, in any crop we reap, 

 must always be the greater the more rapid its general vegetation has 

 been. 



It is a fact familiarly known to all of you, that, besides those circum- 

 stances by which we can perceive the special functions of any one or- 

 gan to be modified, there are many by which the entire economy of the 

 plant is materially and simultaneously affected. On this fact the prac- 

 tice of agriculture is founded, and the various processes adopted by the 

 practical farmer are only so many modes by which he hopes to influ- 



* London and Edinburgh Journal of Science, February, 1840. 



Might not our cheap blue glass be used with advantage in glazing hot-houses, conserva- 

 tories, &c. ? 



t The effect of continued sunshine may be olten seen in our cornfields in May, when, 

 under the influence of propitious weather, the young plants are shooting rapidly up. When 

 such a field is bounded by a lofty hedge running nearly north and south, the ri lijes nearest 

 the hedge on either side will be in the shade for nearly one-half of the day, and will iuviuiit- 

 bly appear of a paler green and less healthy colour. If the hedge be studded with occasion; 1 

 large trees, the spots on which the shadows of those trees rest will be indiciiod hy disiinct 

 pale green patches stretching further into the field than the first, and someiinies Lvcn^lun 

 the second ridges. 



