ORIGIN OF EXDOGKNS FROM EX GENS* 515 



Umbellifera*, in Amaranthm, Phytolacca dioica, species of 

 Actace, Thalictrum f, herbaceous genera of Berberidacea, in some 

 Cycadece, &c. 



One is not prepared to suggest the causes in each case, for not 

 enough is known of the habitats and clitnatal conditions, not 

 only of existing species, but of their ancestors. 



All I would maintain is, that as far as the typical endogenous 

 stem is concerned, it would seem to be probable, from a study of 

 the Xymphceacece above all others, to have been due to the par- 

 ticular degenerating effect of a watery habitat. 



As the monocotyledonous embryo has been considered by 

 several botanists as of a thalloid or less differentiated type than 

 the dicotyledonous, one which I would regard as being more 



degraded, so the stem is looked upon, not as a differential form 

 on a par with that of exogens, but as a degradation from it. Thus 

 Dutrochet wrote : — <; Li theorie que je viens d'exposer place les 

 monocotyledons au dessous des dicotyledons dans les degres de la 

 perfection organique. Les monocotyledons offrent veritablement 

 un arret de formation ; ils se sont arretes a Torganisation gem- 

 maire, qui n'est que transitoire chez les dicotyledons, lesquelsont 

 atteint un degre plus eleve de l'orgauisatiou vegetale " +. 



This arrest and, so to say, degenerated disorganization I would 



attribute primarily to an aquatic medium. 



Foliage of Victoria rkgia and Sagittaria sagitt^folia 

 compared. — Although the typical character of submerged leaves 

 of exogens is to be dissected with filiform segments and that of 

 endugens to be ribbon-like, the first leaves of some of the Nym- 

 pliceacece when germinating bear a remarkably close resemblance 

 to the forms of the leaves of Sayittaria which these latter assume 

 according to the depth of the water. Thus, in a germinating 

 plant of Victoria rtgia the first leaf is altogether deprived of a 

 limb, being reduced to a petiole with a median nerve ; the second 

 has a lanceolate limb, the third being hastate, the fourth sagit- 

 tate-peltate. The subsequent perfect leaves are orbicular-peltate. 



Similarly with Sayittaria sagittcefolia there are ribbon-like 

 phyllodes in deep water, the plant when bearing them alone 

 being known as the form Vallisneriifolia. Then follow conseeu- 



De Bary's Coinp. Anat. &c. p. 253. 

 t Ibid, for details of the structure of the bundles. 



{ " Observations sur la forme et la structure primitives des embrjons 

 vt-gctaux," p. 165. Nouv. Aun. du Mus. d'Hist. Nat, torn. iv. 1835. 



