EXOGENS AND ENDOGENS. 43 



separate them ; and in Endogens they are all mixed together, 

 in consequence of the manner of growth of those plants not 

 requiring the same kind of arrangement of parts as is indispen- 

 sable in Exogens.* This wiU be sufficiently illustrated by. 

 the comparison of the stems of an Oak, a Cabbage, and an 

 Asparagus. 



Tubers, the root-stock of the Iris and Ginger, what are 

 called the roots (corms) of the Colchicum and Crocus, are all 

 so many different forms of stem. 



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-bud, capable of forming other stems or branches 

 like that on which it appeared ; and each is protected and 

 nourished by a leaf which springs from the bark immediately 

 below the bud. Such leaf-buds are the parts that enable a 

 stem, when reduced to the state of a cutting, to produce a new 

 individual like itself; and, without them, propagation by 

 portions of the stem is, under ordinary circumstances, impos- 

 sible. 



Leaf-buds are capable, under fitting circumstances, of 

 growing when separated from their mother branch, whether 

 they are planted in the earth, or inserted below the bark of a 

 kindred species. In the former case, they emit roots into the 

 soil ; in the latter they produce wood, which adheres to the 



* As tliiB -wort excludes everything botanical that does not directly bear upon horti- 

 cultural purposes, I have not explained the difiference between Exogens and Endogens ; 

 wishing the reader to refer for information upon such points to works upon pure 

 botany. 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, apd 

 that they derive their names from the peculiarity of their manner of growth. Exosens 

 (literally, oviside-growers) jaie 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. Endosehs (literally, inside-growers) are plants 

 whose woody matter is augmented annually by internal additions to their centre ; and, 

 consequently, they are continually pushing to their circumference the woody substance 

 formed in previous years. 



