GROWTH OF PLANTS. 153 



We will now consider the manner in which the tree increases 

 in height. A seed germinates ; the plume rises ; the liber, by 

 the vegetative power is urged upward ; but in developing, it 

 gradually becomes less capable of extension ; at length, when 

 it is converted into wood, its growth ceases. The layer of 

 wood then exhibits the form of an elongated cone ; at the sum- 

 mit of the cone a bud is formed, from which a new shoot issues ; 

 a new liber organizes upon the surface of the cone ; this new 

 liber in turn, becomes a woody cone covering the one first 

 formed ; and thus the tree goes on increasing in height and hi 

 diameter. The terminal bud is formed each successive year. 

 After a hundred years of vegetation a hundred cones might be 

 found boxed into each other inthe manner first described*; the 

 spaces comprised between the summits of the cones would 

 show the succession and elongation of the annual shoots. 



As the Wood is formed by the conversion of cambium into 

 alburnum, so from the same liquid, the inner layers of bark are 

 formed, to renew the waste occasioned by the destruction of 

 the epidermis. While the wood is growing externally, that is, 

 at an increasing distance from the centre, the bark is forming 

 internally, and the new layers are pressing outward. 



The growth of trunks, which we have hitherto considered, 

 has relation only to woody plants, but there is a marked differ- 

 ence in the growth of plants, which seems to originate in the 

 peculiar formation of their seeds. 



Between plants which grow from seeds with one cotyledon, 

 and such as grow from seeds with two cotyledons, there is a 

 great difference as to the mode of organization. 



The first kind of plants are called monocotykdonous ; the 

 second, dicotyledonous. Their stems, on account of their dif- 

 ferent modes of growth, have been distinguished into endogenous, 

 signifying to grow inwardly ; and exogenous, signifying to grow 

 outwardly. The discovery of the different modes of growth 

 in these two great divisions of plants, is of recent origin, and 

 constitutes an important era in vegetable physiology. 



The stems of monocotyledons, or endogenous plants have sel- 

 dom a bark distinct from the other texture ; they have no 

 liber, or alburnum disposed in concentric layers ; they have no 

 medullary rays ; and their pith, instead of being confined to 

 the centre of the stem, extends almost to the circumference. 



Advance of the tree in height Difference in the growth of wood and bark 

 Kemarkson the different organization of plants Monocotyledonous plants 

 Why called endogenous Exogenous plants Describe the stem of a mono- 

 cotyledonous plant. 



