CHEMICAL R.VY3 IN TIIE SUN-BEAM. 99 



removed from the centre. It is this transference of the vital circula- 

 tion to newer and more perfect vessels that enables the tree to grow and 

 blossom and bear fruit through so long a life. In animals the vessels 

 are gradually worn out by incessant action. None of them, through 

 old age, are permitted to retire from the service of the body — and the 

 whole system must stop when one of them is incapacitated for the 

 further performance of its appointed duties. 



In regard to the chemical functions of the stem, it is obvious that they 

 are not assigned to the mere woody matter of the vessels and cells. 

 They take place in these vessels, but the nature and extent of the chemi- 

 cal changes themselves must be dependent upon the quantity and kinds 

 of matter which ascend or descend in the sap. The entire chemical 

 functions of the plant, therefore, must be dependent upon and must be 

 moditied by the nature of the substances which the soil and the air re- 

 spectively present to the roots and to the leaves. 



• IV. In describing the functions of the leaf, I have already had occa- 

 sion to advert to the greater number of the circumstances by which the 

 discharge of those functions is most materially affected. We have seen 

 that the purposes served by the leaf are entirely different according as 

 the sun is above or below the horizon ; that the temperature and mois- 

 ture of the air may indeed materially influence the rapidity with which 

 its functions are discharged — but that the light of the sun actually deter- 

 mines their nature. Thus the leaf becomes green and oxygen is given 

 off* in the presence of the sun, while in his absence carbonic acid is dis- 

 engaged, and the whole plant is blanched. 



How necessary light is to the health of plants may be inferred from 

 the eagerness with which they appear to long for it. How intensely* 

 docs the sun-flower watch the daily course of the sun, — how do the 

 countless blossoms nightly droop when he retires, — and the blanched 

 plant strive to reach an open chink through which his light may reach 

 it!* 



That the warmth of the sun has comparatively little to do with this 

 specific action of his rays on the chemical functions of the leaf, is illus- 

 trated by some interesting experiments of Mr. Hunt, on the effect of 

 rays of light of different colours on the growing plant. He sowed cress 

 seed, and exposed different portions of the soil in which the seeds were 

 germinating, to the action of the red, yellow, green, and blue rays, 

 which were transmitted by equal thicknesses of solutions of these seve- 

 ral colours. " After ten days, there was under the blue fluid, a crop of 

 cress of as bright a green as any which grew in full light and far more 

 abundant. The crop was scanty under the green fluid, and of a pale 

 yellow, unhealthy colour. Under the yellow solution, only two or three 

 plants appeared, but less pale than those under the green, — while be- 

 neath the red, a few more plants came up than under the yellow, though 

 they also were of an unhealthy colour. The red and blue bottles being 

 now mutually transferred, the crop formerly beneath the blue in a few 



* A potato has been observed to grow up in quest of light from the bottom of a well 

 twelve feet deep— and in a dark cellar a shoot of 20 feet in length has been met with, the 

 extremity of which had reached and rested at an open window. In the leaves of blanched 

 vegeiables peculiar chemical compounds are formed. Thua in the stalk of the potato a 

 poisOTioq^ substance called aolanin is producsd, which disappears again when the stalk is ex- 

 posed to the light and becomes green. 



