DEFINITIONS AND DESCRIPTIONS. 



59 



ward. The vessels of the wood, Fig. 12. 



like the roots appear to possess 



the power of discrimination, as to 



what substances they will receive. 



When, for example, the trunks of 



several trees, of the same species, 



are cut off above the roots, and 



immersed in solutions of different 



substances, some of these solutions 



will quickly ascend in the tubes, 



and penetrate the entire mass, 



while others will not be admitted 



at all, or very slowly, by the vessels 



of the tree. The functions of the 



stem are performed mostly by the 



alburnum, or sap-wood. 



The branches or twigs are ex- 

 tensions of the trunk, as a h, 

 Fig. 12. 



S. Leaves are still further extensions of the wood, and of the 

 bark. The fibres of the leaves are minute tubes of Avoody mat- 

 ter, connected with the w^ood, from which they receive the sap. 

 The green part of the leaf is an expansion of the bark. The 

 sap descends from this part into the bark, and thence to the 

 root. Hence, the leaf consists of two layers of veins or fibres, 

 covered by a thin membrane, (the epidermis,) which is an ex- 

 pansion of the outer bark. This membrane is filled with small 

 apertures, for the absorj)tion and transpiration of gaseous 

 and liquid bodies. These pores (stomata) on the upper surface 

 are supposed to exhale, and those on the under side of the leaf 

 to inhale substances. They will not absorb all bodies indis- 

 criminately, for they drink in oxygen, carbonic acid and water, 

 but reject the nitrogen. 



4. The fiower-leaves are called petals, and perform the office 

 of inhaling and exhaling various substances; but they absorb 

 oxygen at all times, both day and night, and constandy emit 

 carbonic acid; while, during the day, the leaves absorb carbonic 

 acid and emit oxygen gas, and, during the night, reverse the pro- 

 cess. The flower-leaves also exhale odoriferous particles, the 

 nature of which, it is difficult to determine. 



Although plants differ from animals in giving no signs of 

 perception and voluntary motion, yet in their organs and pro- 

 cesses of nutrition there is a striking analogy. 



