28 VITAL ACTIONS. 



it descends through the liber, in part passing off hori- 

 zontally towards the centre through the medullary 

 rays. Wherever it passes it deposits a portion of its 

 solid parts ; and, consequently, that portion of the 

 wood, namely, the oldest or the heart- wood, through 

 which it has passed the most frequently, will have the 

 greatest quantity of matter accumulated within it, 

 independently of all other reasons for its hardening. 



50. The stem of a plant consists, then, of the fol- 

 lowing parts, viz. : 1. Wood, the oldest of which is 

 heart- wood, and the newest alburnum ; and this is the 

 substance through which sap ascends : 2. Bark, the 

 external coating, down the liber or inner face of which 

 sap descends : 3. Pith, a central portion of the hori- 

 zontal system : and, 4. Medullary Bays, serving to 

 connect the rind with the pith, to hold all the parts 

 together, and to maintain a communication between 

 the centre and the circumference of a stem. The 

 stems of all plants have these four parts more or less 

 evident. They are most visible in European trees or 

 shrubs, in any of which they can be distinctly ob- 

 served ; they are least apparent in annual and herba- 

 ceous plants, because their lines of separation are not 

 defined, all the four parts adhering to each other so 

 firmly as to render it difficult to separate them ; and 

 in Bndogens they are all mixed together, in conse- 

 quence of the manner of growth of those plants not 

 requiring the same kind of arrangement of parts as is 

 indispensable in Exogens.* This will be sufficiently 



* Aa this -work excludes every thing botanical that does not 



