20 



The Garden. 



horizontal system of the wood — in botanical language, the 

 medullary rays. 



The larlc consists originally of cellular tissue alone, but after- 

 ward the inner portion next the wood has the woody tissue 

 formed in it, and becomes the libei\ or fibrous inner bark. 



Whenever a stem is wounded, it is the cellular or horizontal 

 system Avhich forms granulations that eventually coalesce into 

 masses, within which the woody tissue is subsequently devel- 

 oped, and the communication between the two sides of an inci- 

 sion effected. In cuttings, the callus which forms at the end 

 placed in the ground is tlie cellular or horizontal system pre- 

 paring for the woody fibers, which are to pass downward in 

 the form of roots. 



The description m'C have given of the structure of a stem 

 applies to all plants whoso woody matter is augmented annually 

 by external additions below tlio liber, and which are called exo- 

 gensy or outside growers. All the trees and shrubs of the 

 United States, except the few palms of our Southern confines, 

 belong to this class. In the palms, which belong to the class 

 of endogem^ or inside 

 growers, the woody 

 matter is augmented 

 annually by internal 

 additions to their cen- 

 ter, thus constantly 

 BxoGKN. pushing the woody 



growth of former years to their circumference. The stem of 

 the asparagus exhibits a similar structure in an herb. In endo- 

 gens, the cellular and fibrous systems are all mixed together, 

 their mode of growth not requiring the same arrangement of 

 parts as exists in the cxogcns. 



Stems, during their growth, form on their surfaces minute 

 vital points, each of which becomes, or may become, a leaf- 

 bud, capable of forming another stem or branch like that on 

 which it is found. These buds appear immediately above the 

 point of union between the leaf and the stem, and are not. 



^vI3P 



ENDOGEN. 



