MANUAL OF NATURAL HISTORY. 389 



without, which from year to year gives the look of 

 a series of concentric rings in a transverse section of 

 the trunk. The centre is occupied by a lighter tis- 

 sue, named "pith/" while the exterior is covered 

 with bark, which is renewed annually by fresh 

 matter from within, or on that surface which is in 

 contact with the true wood. The appearance of a 

 cross section of a stem presents, as just mentioned, 

 concentric rings, varying in number according to 

 the age of the individual, which are again crossed 

 at right angles, by lines radiating from the centre. 

 To this, however, there are many exceptions ; in 

 some the radii are wanting, in others there is but a 

 single ring, or there may be irregular layers of cel- 

 lular tissue between the woody zones. These have 

 been proposed by Lindley to be placed in a separate 

 class, to which the name of " Homogens " has been 

 assigned, but this idea, though apparently founded 

 on sound principles, has not yet been worked out. 

 Exogens were by De Candolle styled Dicotyledones, 

 from the seed being composed of two cotyledons. 

 The different parts of the flower are generally either 

 five or some multiple of that number ; occasionally, 

 however, four is the primary number, and in some 

 rare cases three. 



Various methods have been proposed for arrang- 

 ing this class. Some adopt as the basis of their 

 division the existence or non-existence of distinct 

 floral envelopes, the former being divided into 

 monopetalous and polypetalous. Slightly differ- 

 ing from this is the plan of De Candolle, who 



