GROWTH OF MONOCOTYLEDONS. 117 



height. A seed germmates ; the plume rises ; the cambium, in de- 

 veloping, gradually becomes less capable of extension ; at length, 

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

 wood then exhibits the form of an elongated cone ; at the summit of 

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

 layer of alburnum organizes upon the surface of the cone ; this, in 

 turn, becomes perfect wood, covering the layer first formed ; and 

 thus the tree goes on increasing in height and in diameter. The ter 

 minal bud is formed each successive year. After a hundred years 

 of vegetation, a hundred cones might be found boxed within each 

 other in the 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 albur- 

 num, 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 gi'owing externally, that is, at an increasing dis- 

 tance from the centre, the bark is forming internally, and the new 

 layers are pressing outward. 



Growth of Monocotyledonous Plants. 

 The growth of trunks, as hitherto considered, has relation only to 

 woody plants ; but between plants which grow from seeds with one 

 cotyledon, and such as grow from seeds with two cot5dedons, there 

 is a great difference as to the mode of organization and growth. 



The first kind of plants are called monocotyledonous ; the second 

 dicotyledonous. Their stems, on account of their different modes of 

 growth, have been distinguished into endogenous, signifying to grow 

 inwardly ; and exogenous, signifying to grow outwardly. The dis- 

 covery of the different modes of growth in these two great divisions 

 of plants, is of recent origin, and constitutes an important era in ve- 

 getable physiology. 



The stems of monocotyledonotts or endogenous plants have seldom 

 a bark distinct from the other texture ; they have no liber, or albur- 

 num 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. 



The wood is divided into fibres running 

 longitudinally through the stem, (see Fig. 

 119, where the dots represent the fibres;) 

 each of these fibres seems to vegetate sepa- 

 rately ; they are ranged around a central 

 support, and are so disposed that the oldest 

 are crowded outwardly by the develop- 

 ment of new fibres in the centre of the 

 stem; this pressure causes the external 

 layers to be very close and compact. 

 This mode of increase, httle favoipable to 

 growth in diameter, produces long and 

 straight stems, nearly uniform in size 

 throughout their whole extent; as the 

 palms and sugar-canes of the tropics, and 

 the Indian corn of our climate. Most of these plants present us with 

 roots of the fibrous kind. 



Describe the manner in which the tree increases in height/?— Difference in the 

 growth of wood and bark— Remarks on the different organization of plants— Mono- 

 cotyledonous plants— Why called endogenous?— Exogenous plants— Describe the 

 stem of monocotyledonous or endogenous plants— Describe the stem of a monoeoty* 

 ledonous plant. 



