48 COMPOUND OEGANS OP PLANTS. 



genous, and present a striking contrast to the exogens of tem- 

 perate latitudes. A tall, cylindrical and unbranched stem rises 

 to the height of from 100 to 150 feet, crowned at the summit 

 with a magnificent cluster of leaves, many feet in length, bend- 

 ing elegantly downwards, and presenting altogether one of the 

 most graceful and beautiful objects that can adorn a landscape. 



The Uxogenous Stem. — Since the exogenous class of plants 

 is by far the largest in every part of the world, and embraces 

 all the trees and shrubs with which we are familiar in northern 

 climates, the structure of this kind of stem (iemands a more 

 detailed and particular investigation. Every exogenous stem 

 presents, on the cross-section, an arrangement of matter into 

 three parts, called, respectively, the bark, the wood, and the 

 pith. To obtain, however, a clear idea of the origin of 

 the exogenous stems, it is necessary to follow the course of 

 the development of the stem from the embryo state. 



The first year's growth. — If we place a seed in the ground 

 at the temperature of 32° Fahr., it will remain inactive until 

 it finally decays; but, if the earth be moist and above the 

 temperature of 32°, and the seed be effectually screened from 

 the action of the light, its integuments will gradually imbibe 

 moisture, soften, and swell, oxygen will be absorbed, carbonic 

 acid expelled, and the vital action of the embryo will com- 

 mence. It now elongates downwards into the earth by its 

 radicle, and upwards into the air by its plumule, or young 

 stem, lifting the cotyledons above the earth's surface. The 

 cotyledons thus elevated acquire a green color, by the deposi- 

 tion of carbon absorbed from the atmosphere under the influ- 

 ence of solar light, and ultimately assume the form of two 

 opposite leaves. The process of germination is now completed, 

 and the root, stem and leaves being formed, we have a simple 



