ORIGIN OF ENDOGENS FROM EXOGENS. 401 



seen everywhere, both morphologically aud histologically*; and 

 the object I have in view is to show that many of the characters 

 common, more especially, to the vegetative organs of endogens, 

 and regarded as points of affinity, are just those which are cha- 

 racteristic of the adaptations of exogens to an aquatic, and, in 

 some cases, to other habits of life. 



Points of Difference between Endogens and Exogens. 



The ^Embryo. — The first difference of importance lies in the 

 structure of the embryo, which has supplied the names " Mono- 

 cotyledons " and 9t Dicotyledons " to the two Classes ; just as 

 the structure of the stems furnishes those of " Endogens " and 

 w Exogens." These I shall consider hereafter. 



The first observation to be made is that the development of 

 the embryo from the egg-cell to the proembryonic condition is 

 the same in both dicotyledons and monocotyledons ; i. e. up to 

 the period, and even beyond it, of the quadrature of the terminal 

 cell. The visible differences begin when the globular proembryo 

 becomes unsymmetrical in monocotyledons. With regard to the 

 origin of the single cotyledon in the latter, M. Ph. van Tieghem 

 observes f that, of the two cells formed by the first longitudinal 

 partition of the embryo-cell at the end of the suspensor, if they 

 are equal in size they lay the foundation of a dicotyledonous 

 embryo, but if they be unequal then the larger one only becomes 

 a cotyledon. 



Judging, however, from the numerous figures of proembryos 



* It must be borne in mind that there is no fundamental difference between 

 the two classes of characters "adaptive " and those indicating assumed " affinity/' 

 It was my contention elsewhere (' Origin of Floral Structures/ &c.) to prove 

 that characters of flowers usually regarded as indicating a common descent, in 

 all cases owe their origin to adaptations to the environment, including the 

 visits of insects; then, when plants have once established their affinities, which 

 are mainly based upon their reproductive organs, other and similar adaptations 

 may appear in both the reproductive and vegetative organs, often in widely 

 different orders. These, therefore, cannot be called characters of affinity. 

 Thus, for example, we find a similar labiate corolla with didynamous stamens 

 in more than one order : the form of a pea-blossom is mimicked by species of 



Pelargonium, Polygala, and even Collinsia ; so, too, a pea and a Bignonia have 

 foliar tendrils ; air-canals with perforated diaphragms occur in aquatic exo- 

 gens and endogens exactly alike, as well as floating leaves with stoxnata above 

 and none below, as in Ranunculus heterophyllus, Hydrocharis, and Alisma 

 nutans, &c. ; but not one of these can be regarded as indicating affinity, they 

 are adaptations alone. 



t Ann. des Sci. 'Sat. 5 ser. torn. xiii. p. 24. 



