46 MORPHOLOGICAL DEVELOPMENT. 



of parts, have taken place. Seeing how, in an individual 

 plant, the single leaves pass into compound leaves, by the devel- 

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

 to simulate axes ; and seeing that leaves ordinarily exhibit- 

 ing definitely-limited developments, occasionally produce 

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

 sibility of still greater changes in foliar organs. When, fur- 

 ther, we find that within the limits of one natural order, 

 petioles usurp the functions and appearances of leaves, at the 

 same time that in other orders, as in Jluscus, lateral axes so 

 completely simulate leaves that their axial nature would never 

 have been supposed, did they not bear flowers on their mid- 

 ribs or edges ; and when, among Cactuses, we perceive that 

 such metamorphoses and re-metamorphoses take place with 

 great facility ; our suspicion that the morphological elements 

 of Phsenogarus admit of profound transformations, is 

 deepened. And then_, on discovering how frequent are the 

 monstrosities that do not seem satisfactorily explicable without 

 admitting the development of foliar organs into axial organs ; 

 we become ready to entertain the hypothesis, that during the 

 p. volution of the phsenogamic type, the distinction between 

 leaves and axes has arisen by degrees. 



"With our pre-conceptions loosened by such facts, and 

 carrying with us the general idea which such facts suggest, 

 let us now consider in what way the typical structure of a 

 flowering plant may be interpreted. 



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

 the structures of Endogens and Exogens, in the structures 

 of those inferior plants that approach to them — Acrogens. 

 The various divisions of this class present, along with sundry 

 characters which ally them with Thallogens, other charac- 

 ters by which the phsenogamic structure is shadowed forth. 

 While some of the inferior Hepaticce or Liverworts, severally 

 consist of little more than a thallus-like frond ; among the 

 higher members of this group, and still more among the 



