PHYSIOLOGY OF VEGETABLES. US 



lace that leaves perspire, and absorb the principles of 

 the atmosphere necessary for vegetation. The under 

 surface is so constructed that water will not wet it, but 

 collects in large drops and rolls off, otherwise its func- 

 tions might be obstructed. 



There are in the leaves of some plants, either on one 

 or both surfaces, a large number of small whitish 

 points, scarcely apparent to the naked eye but easily 

 distinguished with a glass. They are found to consist 

 of small fissures surrounded by areas. These pores, 

 which contain air only, are surrounded by a pair of 

 cells, which contain a greenish fluid in common with 

 the other cells of the leaf. Through these pores and 

 cells the communication appears to be kept up between 

 the external air and the juices of the leaf. See Smith's 

 Bot. Note 84, by Prof. Bigelow. 



The upper side of leaves is uniformly turned to 

 the light. If a branch be overturned, the foot-stalk 

 will twist until the leaf regains its former position. 



Light is so essential to vegetation that few plants can 

 be perfect without it. Plants growing in the shade, 

 besides other imperfections, are without colour. When 

 they are placed in a room where the light comes only 

 laterally from a single window, they incline not only 

 the leaves, but the whole plant takes a course to- 

 wards it. 



Three kinds of vessels centre in the lungs of animals, 

 the Arteries, the Veins, and the dir-vessels of the Wind- 

 pipe. In tbe leaves, the Sap-vessels, the CorticM-ves- 

 sels, and Pores, which convey air, communicate with 

 each other. 



In the lungs of animals the purple blood from the, 

 veins absorbs oxygen gas from the air inhaled through 

 the air-vessete of the Wind-pipe, which combines with 

 a portion of the carbon of the blood, and is thrown off 

 at the next breath in the form of carbonic acid gas, 

 while the caloric of the oxygen is absorbed, to be car- 

 ried on in the arteries and disengaged in the extreme 

 vessels. Watery vapour is also given off by the Junes. 

 10* 



