ORI0JN OF EKDO0EITS FROM EXOGENS. 501 



It may be added here that a plexus is found in the Piperacea, 

 which also have medullary cords exactly as in Commelyna. 



Origin of the Coleorhiza. — Now, not only is there a signifi- 

 cance in the existence of sheathing petioles in aquatic exogens, 

 which thus appear to point to the origin of this prevailing 

 feature in endogens, but it may supply the interpretation of 

 the coleorhiza in the embryo of Grasses, &c. ; for the sheathing 

 base of the leaf is not only characteristic of the fully developed 

 condition, but would seem to be also applicable to the cotyledon ; 

 or, conversely, what commences in the embryo is continued up 

 in the leaves. In Grasses, though the sheath of the true leaves 

 is partially split, unlike that of the Cyperacece, yet the sheath of 

 the first leaf, formed without a blade, is entire. This is what 

 Mirbel called the " pileole." Now the broad basal attachment 

 of the cotyledon (and perhaps the homologue of the sheath of 

 the opposite cotyledon may be present as well) and the axial 

 root are all fused together at the region of the insertion of the 

 cotyledon (shield). The root differentiates itself below, sepa- 

 rating away from the sheathing " coleorhiza," which it finally 

 pierces at the basal end, and thus becomes endogenous. On the 

 other hand, the plumule is generally included in a fold of the 

 cotyledon or else in a sort of tube formed by the basal part of it, 

 called by Mirbel the " coleoptile " ; or, again, it may lie between 

 the lobule and cotyledon, which are flat as in the oat, just as in a 

 dicotyledon. 



I would thus suggest, but purely hypothetical!)', that the 

 coleorhiza of certain endogens, as of Grasses, may represent 

 deeurrent sheaths of the two cotyledons united; or, perhaps, of 



cTIntroduction a un travail Anatomique, Physiologique et Botanique sur Ja 

 Pamille des Labiees " (pp. 8, 9):— "La plupart des caracteres de l'anatomie 

 comparee des vegetaux se doivent tirer de certains rapports entre la disposition 

 dea vaisseaux nourriciers et la marehe des developpemens .... Ainsi, pour citer 

 quelques exemples, les filets vaseulaires disposes a la circon fere nee des jeunes 

 tiges monocotyledones produisent necessairement des feuilles en gain a a tea 

 dont on reconnoit le type dans Funite de cotyledon et dans la structure de cet 

 or^ane; et, par une consequence naturelle, 1' unite de cotyledon est l'indice de 

 Tengainement des feuilles, et la forme de ceiles-ci revele 1'organisation des 

 tiges. Ainsi, les tiges dicotyledones a feuilles engainantes ont une ecoree 

 organises a la maniere des tiges monocotyledones [On peut voir facilement 

 cette organisation dans la plupart des Ombelliferes (note)], et ces dernieres, 

 lorsque leurs filets viennent a se reunir en couehe, produisent, au lieu de 

 feuilles engainantes, des feuilles retrecies a leur base." 



