ORIGIN OF ENDOGENS FROM EXOGENS. 499 



l 



Their idea of regarding the embryo of Trapa in the light of 

 a thallus is paralleled by the view of Agardli with regard 

 to monocotyledons generally. This author raised the question 

 whether it was desirable to regard the embryo of mono- 

 cotyledons and of dicotyledons as very distinct, and came to the 

 conclusion that a monocotyledonous embryo was a degraded state 

 of a dicotyledonous one ; and indeed he uses the same expression 

 " thalloid " for it as now proposed by Drs. Gibelli and Ferrero. 

 He thus wrote: — a Verumin iis quoque Monocotyleis embryis, in 

 quibus pars cotyledonaris a radiculari quam maxime distincta 

 videretur, hae partes vix iis dici possunt vicibus fungi, quse in 

 Dicotyleis embryis horum organorum propria sunt. Si igitur 

 perfectissima etiam monocotylea embryaeum evolutionis gradum 

 proximo tenere videntur, qui in imperfectioribus quibusdam 

 Dicotyleis embryis cernitur, nonne verius est statuere, Mono- 

 cotyledones potius eas formas esse, quae propter imperfectam 

 embryi formationem in inferiori quodam gradu constiterint, 

 quam proprium quendam et Dicotyleis parallelum plantarum 

 typum?"* 



Origin of the Sheathing Petiolef. — A well-nigh universal feature 

 of the leaves of endogens is their sheathing the stem in various 

 degrees and not infrequently completely surrounding the latter, 

 as occurs in palms, Cyperacece, Ac. This is characteristic not 

 only of endogens but also of many aquatic exogens, as of Um- 

 bellifercBy Hanunculacece, &c. In an ordinary leaf of any exogen 

 with a narrow and stiff petiole, there is either a single central 

 cord or midrib, or else associated with it at least one pair of 

 lateral cords which have arisen from points in the axial xylem- 

 cylinder at some distance right and left of the central one. 

 When a leaf has a sheathing base, however, there are generally 

 present a larger number of lateral cords departing from points 

 distributed more or less over the whole circumference and 



* ■ Theoria Systematis Plantarum/ per Jac. Geo. Agardh, 1858, p. lxxxiii. 

 We shall see below that Dutrochet came to a similar conclusion for the struc- 

 ture of the stem of endogens ; and I shall give reasons for drawing a like 



inference from the study of the foliage. 



t This section should follow later when treating of leaves, but I insert it 

 here, as I wish to endeavour to account for the coleorhiza or sheath over the 

 radicle of endogens. It also forms the pileole or first leaf-sheath of the plu- 

 mule of grasses. See Van TieghenVs paper, "Observations anatomiques sur 

 le Cotyledon des Graminees" (Ann. des Sci. Nat., Bot. 5 ser. torn. xv. 1872, 

 p. 236, pla. 13, 14). 



