GEOWTH BY THE STEM. 29 



illustrated by the comparison of the stems of an Oak, 

 a Cabbage, and an Asparagus. 



51. Tubers, the root-stock of the Iris and Ginger, 

 what are called the roots (cormi) of the Colchicum 

 and Crocus, are all so many different forms of 

 stem. 



52. It is the property of a stem, during its growth, 

 to form upon its surface, at irregularly increasing or 

 diminishing distances, minute vital points of the same 

 nature as that in which the stem itself originated. 

 Each of those points becomes, or may become, a leaf- 

 directly bear upon horticultural purposes, I have not explained the 

 difference between Exogens and Endogens ; wishing the reader to 

 refer for information upon all such points to works upon pure bo- 

 tany. Nevertheless, as these words are of frequent occurrence, I 

 may as well state that they denominate the two greatest classes in 

 the vegetable kingdom, to one or other of which almost all the 

 flowering plants of common occurrence are referable, and that they 

 derive their names from the peculiarity of their manner of growth. 

 Exogens (literally outside-growera) are plants whose woody matter is 

 augmented annually by external additions below the liber, and, 

 consequently, they are continually enclosing within their centre the 

 woody substances formed in previous years ; to such plants, a lateral 

 communication between the centre and the circumference, by means 

 of medullary rays, seems necessary. Endogens (literally inside- 

 grmeers) are plants whose woody matter is augmented annually by 

 internal additions to their centre ; and, consequently, they are con- 

 tinually pushing to their circumference the woody substance formed 

 in previous years. 



[All the trees or shrubs of the United States, except the few Palms 

 of our southern confines, are examples of Exogens : the Palms afford 

 the best example of Endogens, while the stem of an Asparagus ex 

 hibits a similar structure in an herb. A. G.] 



