4:6 



MORPHOLOGICAL DEYELOrMENT. 



of parts, have taken place. Seeing how, in an individui; 

 plant, the single leaves pass into compound leaves, by thedeve] 

 opment of their veins into mid-ribs while their mid- ribs begi 

 to simulate axes ; and seeing that leaves ordinarily exhibil 

 ing definitely-limited developments, occasionally produc 

 other leaves from their edges ; we are led to suspect the poi 

 sibility of still greater changes in foliar organs. When, fui 

 ther, we find that within the limits of one natural orde: 

 petioles usurp the functions and appearances of leaves, at th 

 same time that in other orders, as in Ituscus, lateral axes s 

 completely simulate leaves that their axial nature would nev« 

 have been supposed, did they not bear flowers on their mic 

 ribs or edges ; and when, among Cactuses, we perceive th: 

 such metamorphoses and re-metamorphoses take place wit 

 great facility ; our suspicion that the morphological elemem 

 of Phsenogams admit of profound transformations, 

 deepened. And then^ on discovering how frequent are tt 

 monstrosities that do not seem satisfactorily expKcable -ivithoi 

 admitting the development of foliar organs into axial organs 

 we become ready to entertain the hypothesis, that during tt 

 evolution of the phasnogamic type, the distinction betwee 

 leaves and axes has arisen by degrees. 



With our pre-conceptions loosened by such facts, an 

 carrying with us the general idea which such facts sugges 

 let us now consider in what way the typical structure of 

 flowering plant may be interpreted. 



§ 192. To proceed methodically, we must seek a clue 1 

 the structures of Endogens and Exogens, in the structun 

 cf those inferior plants that approach to them — Acrogeu 

 The various divisions of this class present, along with sundr 

 characters which ally them with Thallogens, other charac 

 ters by which the phaenogamic structure is shadowed fort! 

 While some of the inferior Hepaticm or Liverworts, several! 

 consist of little more than a thallus-like frond ; amonff t\ 

 higher members of this group, and still more among tl 



