OBIGIN OF ENDOGENS PROM EXOGENS. 507 



The consequence is that the primary root, which must be sup- 

 plied with nourishment from the first formed leaves, suffers and 

 finally perishes. This is true for aquatics of both classes ; but, 

 having become an hereditary feature, it is now characteristic of 

 all endogens, whether they be terrestrial or aquatic in habit. 



An illustration of the above may be seen in the germination of 

 Trapa natans. One cotyledon is large and furnishes food- 

 material to the primary root ; but the other cotyledon is atrophied. 

 The consequence is that one side of the hypocotyl and root is 

 nourished more than the other. This causes a curvature towards 

 the side on which the atrophied cotyledon lies, and induces the 

 hypocotyl to develop a quantity of adventitious roots on one, 

 the underside, only *. The main or tap-root soon appears 

 to cease growing ; so that the stem gradually assumes the well- 

 known form of an inverted cone, characteristic of Indian corn 

 and other endogens. A similar stem may be seen in aquatic 

 Vmhelliferce. 



The Boot-cap or Pileorhiza. — M. Flahault, as far as I am aware, 

 was the first to discover a marked difference in the development 

 of the root-cap of endogens from that of exogens t, in that it is 

 early separated and grows independently of the initial cells of 

 the rest of the root, in the former; but it is not so in exogens. 



4- 



+ 



M. Ph. van Tieghem has explained how this comes about 

 He contrasts the two kinds of root-cap as follows : — " Chez les 

 Dicotyledones etles Gymnospermes, c'est l'assise la plus externe 

 de l'ecorce qui subit une serie de cloisonnements tangent iels, 

 ordinairement centrifuges, et qui produit ainsi la zone corticale 

 externe, dont l'assise la plus exterieure devient l'assise subereuse, 

 tandis que tout le reste, developpe en direction centripete, forme 

 la zone corticale interne. 



" Chez les Monocotyledones et les JSTympheacees, l'assise corti- 

 cale externe demeure indivise et devient l'assise pilifere ; c'est la 

 seconde assise qui subit le cloisonnement tangentiel centrifuge et 

 produit la zone corticale externe, dont l'assise la plus exterieure 

 devient l'assise subereuse 



" Chez les Dicotyledones, a part les JNTympheacees, chez les 

 G-ymnospermes, chez les Lycopodes et les Isoetes, l'assise la plus 



* See Organ. Veget. par. A. P. de Candolle, torn. ii. pi. 55. 



t " Recherches sur V Accroissement terminal de la Racine chez les Phanero 



games," Ann. des Sci. Nat. 6 ser. vi. 1878. 

 X ' Traite de Botanique,' torn. i. p. 694 seqq. 



