INTRODUCTION TO BOTANY. xxxix 



Many roots are useful articles of food. A sugar is extracted 

 from the beet ; some are useful in dying : many are valuable 

 in medicine. 



Organization of Dicotyledonous Stems. — See 

 Plate 11, 169. and Explanation. 



The trunk of these is formed of concentric layers placed 

 within each other ; it may be compared to a series of tubes 

 set the one within the other, and increasing in magnitude 

 from the centre to the circumference. A transverse section 

 presents the following objects : 1. in the centre, the medul- 

 lary canal, formed of the medullary (pith) tube, which consti- 

 tutes the parietes (side walls of the canal,) and of the pith 

 which occupies its cavity. 2. At the circumference is seen 

 the bark, composed of the epidermis, or that external pellicle, 

 (thin skin,) which covers all parts of the vegetables of the her- 

 baceous integument, of the cortical, (bark,) layers, and of the 

 liber, (true bark.) 3. Between the medullary tube and the 

 bark are found the woody layers, composed externally of 

 alburnum, or false wood ; internally of wood properly so 

 called." See Plate, 1 1. 



§. 1. Of the Epidermis, or Outermost-covering. 

 It is a thin, nearly transparent layer, of an uniform tissue, 

 which appears to consist of cells of variable form : it presents 

 a great number of small openings or pores. Plate, 11, 162. 

 It covers all parts of the vegetable. It tears and cracks, when 

 the trunk has acquired a certain size; it falls off in patches or 

 plates ; for example, hirch, and plane. The true office of 

 cortical pores, says Richard, consists in affording a passage to 

 air. 



§. 2. Of the Herbaceous Integument. 



Under the epidermis is observed this layer of cellular tissue. 

 In young stems its colour is generally green. It covers the 

 trunk, the branches, and their divisions, and fills up the spaces 



' The different parts of tlie stem are described in successiorij proceeding 

 from the circumference to the centre. 



