EXOGENS. 137 



CHAPTER X. 



EXOGENS. 



In all places, then, and in all seasons, 



Flowers expand their light and soul-like wings, 

 Teaching us, by most persuasive reasons, 



How akin they are to human things. 

 And with childlike, credulous affection, 



We behold their tender buds expand, 

 Emblems of our own great resurrection, 



Emblems of the bright and better land. 



LONGFELLOW. 



i. THE term Exogen (from exo, outward, and gennao, 

 to produce) is applied to those plants which produce 

 woody and vascular layers toward the circumference. 

 It is the largest class, or type, in the vegetable kingdom, 

 including about 7,000 genera and 70,000 species of flow- 

 ering plants. 



External to the woody layers, and between them and 

 the bark, is a layer of semifluid mucilaginous matter 

 called Cambium. Its cells are exceedingly delicate. 

 New cells are continually being added, on the inner side 

 of the Cambium layer, to the thickness of the wood, 

 and on the outer side of it, to the thickness of the bark, 

 increasing the diameter of the axis of the plant. 



At the apex of the stem, and at that of the root, the 

 Cambium layer is continuous with the cells which retain 

 the power of dividing in these localities. 



The general appearance of the axis of an exogenous 



plant is that of a double cone; one cone representing 

 12* 



