xlii INTRODUCTION TO BOTANY. 



§. 8. Of the Pith. 



It is that spongy, light substance, composed almost exclu- 

 sively of cellular tissue in its simple state, which fills the medul- 

 lary tube. The pith communicates with the herbaceous cellular 

 layer of the bark, by means of peculiar prolongations, which 

 cross the true wood, (corpus ligneum.) These medullary pro- 

 longations serve to establish a direct communication between 

 the pith, and the external cellular tissue of the stem. The 

 medullary rays exist also in the greater part of the thickness 

 of the bark, and serve to establish a communication between 

 the inner medulla, and the outer; but those of the bark have 

 no direct communication with those of the woody layers. The 

 use of the pith seems uncertain. 



Of the Stem of Monocotyledons. See Plate, 1 1 , and 

 Explanation. 



In this, all the parts seem to be united with each other. 

 The pith occupies the entire thickness of the stem ; the wood, 

 disposed in longitudinal bundles, is, as it were, dispersed with- 

 out order in the midst of the medullary substance. The bark 

 does not always exist, or is very indistinct. 



Organization of the Root. 



All roots are generallv oroanized like the stems : the latter 

 generally grow in height by every point of their extent; roots 

 are lengthened only by their extremity. 



Growth of Vegetables, 



Takes place, as in animals, within outwards, or in other words 

 by intus-susception. In proportion as the height of vegeta- 

 bles increases, their diameter becomes more considerable. 



Of the Growth of Dicotyledonous Trees* 



Of the growth in diameter there are three opinions. 1st. 

 That it is carried on by the annual change of liber into albur- 

 num, of the alburnum into wood, and by the successive 

 renewal of the liber. Such is the foundation of Du Hamel's 



