BOTANY. 201 



Vessels. 



In all plants there exists a system of tubes, or 

 vessels, which at one extremity terminate in roots, 

 and at the other in leaves. 



Organs. 



Every plant displays four systems of organs, or 

 analogous parts, viz. 



1. The root. 



2. The trunk, branches, or stem. 



3. The leaves. 



4. The flower or seed. 



The branch or root of a tree, when cut trans- 

 versely, exhibits three distinct substances : the bark, 

 the wood, and the pith.* 



Germination. 



For a healthy germination are required : 



1 . A due temperature, above the freezing point, 

 and below 100 Fahrenheit. 



2. Moisture in due proportion. 



3. A proper access of air, the oxygen of which is 

 slowly converted into carbonic acid. 



The Sap. 



The sap descending through the bark, after being 

 modified by the leaves, is the principal cause of the 

 growth of the tree. The alburnum is the great vas- 

 cular system through which the sap rises. 



The Wood. 



The wood of trees is composed of an external or 

 living part, called alburnum, or sap-wood, and an in- 

 ternal or dead part, named the heart-wood. 



* Plants breathe like ourselves, but differently by day and 

 night. 



