112 



PHYSIOLOGICAL VIEWS. 



creases in liight. A seed germinates ; tlie plumule rises ; un 

 axis is developed, with its nodes which throw ofl* branches ; the 

 cambium performs its office, but gradually becomes less capable 

 of extension, and when it is converted into wood its circulation 

 ceases. The layer of wood then exhibits the form of an elon- 

 gated 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 hight and in diameter. The terminal bud is 

 formed each successive year. After a hundred years of vegeta- 

 tion, a hundred cones might be found boxed within each other ; 

 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 fluid 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 center, the bark is forming internally, and the 

 new layers are pressing outward. 



Growth of Monocotyledonous Plants. 



135. The growth of trunks as hitherto considered has rela- 

 tion only to woody plants ; — but between plants which grow 

 from seeds with one cotyledon, and such as grow from seeds 

 w4th two cotyledons, there is a great difference as to the mode 

 of organization and growth. Their stems, on account of their 

 different modes of growth, have been distinguished into endo- 

 ge7ious, 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 constitutes an im- 

 portant era in vegetable physiology. 

 The stems of inonocotyledonous or en- 

 dogenous plants have seldom a bark 

 distinct from the other texture ; they 

 have neither liber nor alburnum dis- 

 posed in concentric layers ; they have 

 no medu.llary rays ; and their pith, in- 

 stead of being confined to the center 

 of the stem, extends almost to the cir- 

 cumference. The wood is divided into 

 fibers runnino; longitudinally through 

 the stem (see Tig. 126, where the dots 



134. Describe the manner in which the tree increases in hight— Difference in the growth of wood 

 an<l bark.— 135. Difference in the growth of plants of the two great classes— Describe the growth of 

 the endogenous stem. 



