xliv Classification. 



into two lobes (cotyledons). At the hinge or point of junction 

 of the two lobes may be seen the radicle, or incipient root, and 

 the plumule, or incipient bud. A pea or bean or the seed of 

 any large leguminous tree will give as good an example of this 

 as can be, but the skin must be taken off carefully and the 

 embryo (the inner part of the seed) handled tenderly. In 

 small seeds it is difficult to distinguish the parts except with a 

 microscope. 



The other characteristics of Exogens are, firstly, that the 

 stems in trees or other perennials are composed of pith, fibre in 

 regular concentric layers, and bark ; and secondly, that the 

 leaves have a midrib, from which the veins branch out in all 

 directions, forming a fine network. 



In Monocotyledons or Endogens, on the other hand, the 

 embyro on the skin being removed does not divide into two 

 parts, and the young plant is developed from a sheath-like 

 cavity on one side. A grain of wheat is a sufficient example. 

 Besides this, the stems of endogens are not separable into 

 pith, fibre, wood, and bark, but consist of fibre irregularly 

 imbedded in cellular tissue with a firmly adherent rind out- 

 side. The leaves also, which are, as a rule, simple and entire, 

 have generally no midrib, but simple and parallel veins running 

 longitudinally. 



The most obvious of the above distinctions to the ordinary 

 observer is that of the veins of the leaves, but there are a few 

 genera and species of Monocotyledons which have leaves with 

 netted veins. 



The first great division of Exogens is into polypetalous and 

 monopetalous, i.e. those having a corolla of separate petals or 

 of one piece respectively. 



A. EXOGENS. 



Polypetalous exogens, POLYPETALJ;, are described thus. 

 Plants having the stamens and pistil surrounded within the 

 calyx by separate petals, to which they are not attached. 



But to the above definition there are various exceptions. 



(1.) "When the stamens are monadelphous the petals generally 

 adhere more or less to the staminal tube or column. 



(2.) In some orders, e.g. Olacineae, Ilieinese, etc., the petals 

 adhere slightly at the base. 



(3.) In a few small genera in various orders 'the corolla is 



