506 KEY. GEORGE HENSLOW ON A THEORETICAL 



root, with the subsequent development of adventitious roots in 

 an ascending series, which is, of course, one of the most charac- 

 teristic features of endogens. The stem thus assumes the form 

 of an inverted cone. It is, however, by no means infrequent 

 in aquatic exogens. Thus it is characteristic of NymphceacecB, 

 and is very obvious in aquatic members of TTmbelliferce, such 

 as (Enanthe Phellandrium. It occurs also in CeratopTiyllum 

 demersum. 



Mirbel so long ago as 1810 discovered the cause of the arrest 

 of the primary root in endogens. He says : — " J'ai cherche si la 

 physiologie n'expliquoit pas ce phenomene. J'ai trouve que la 

 radicule correspond au cotyledon, qu'elle est nourrie par lui, et 

 que son deperissement a lieu a l'epoque ou le cotyledon se fletrit. 

 J'ai trouve en outre qu'il existe une relation bien manifeste 

 entre les radicelles laterales et les nouvelles feuilles, en sorte que 

 chaque feuille est en rapport avec une ouplusieurs radicelles." 



The arrest is not confined to the primary root in the Nym- 

 plicEacecp, but is continued into the first formed leaves, which 

 thus pass through various stages of development, assuming in 

 succession the following forms — linear, spathulate, sagittate, and 

 hastate, before that the blade can arrive at the rounded form 

 so characteristic of floating leaves of Water-lilies t, as also of 

 Ltmnanthemwrn, Sydrocharis, &c. 



I think, then, we can perceive with Mirbel the clue to the 

 arrest of the primary or axial root of endogens. Since the 

 development of roots depends upon the foliage, it is easy to see 

 that when cotyledons are raised above ground and act as leaves, 

 they can maintain the existence of the tap-root and enable it to 

 grow until the foliage of the plumule is developed, and so carry 

 on the same work, as in mustard or any other ordinary terrestrial 

 exogen. Again, if the cotyledons remain below ground, the 

 plumule at once develops its leaves perfectly, as in the oak, and 

 its foliage can then nourish the tap-root instead of the cotyledons 

 doing so. 



In submerged aquatic plants, however, the first leaves are 

 more or less rudimentary and arrested in character. Moreover, 

 being under water their assimilating powers are greatly impeded. 



* "Examen de la Division des Vegetaux en Endorhizes et Exorhizes," Ann. 

 Mub. d'Hist. Nat. 1810. 



t See Tr£cul, Ann. Sci. Nat., Bot. 4 ser. i. pi. 12. fig. 1, and Barber, " On a 

 change of Flowers to Tubers in Kt/mphaa Lotus'' Ann. Bot. iv. p. 105, pi. v. 

 fig. 13. 



