ORIGIN OF ENDOGENS FROM EXOGENS. 521 



It will not be amiss to give a few more illustrations. Thus 

 Epipactis latifolia, Cypripedium Calceolus, and Qoodyera repens 

 have leaves with many parallel curvilinear ribs united either by 

 straight or wavy cross-bars with no other reticulations, thus 

 closely resembling such aquatic types as Potamogeton. Listera 

 ovata illustrates a more advanced condition in that, while retaining 

 the curvilinear ribs, the cross-bars are mainly perceptible only 

 on the margin, the rest of the blade being covered with minute 

 ramifications. Iu this plaut the cells of the epidermis have the 

 deeply waved margins so common in exogens. Conversely, it 

 may be mentioned here that Hippuris, with its elongated ribbon- 

 like submerged foliage, has elongated rectangular cells like the 

 prevailing ones of the epidermis of most endogens. Again, 

 Alisma Plantago y var. lanceolatum, and A. Plantago, proper, have 

 equal-sized longitudinal rita, but they are connected by cross- 

 bars at an angle of 45°; while the latter has the parallelograms 

 filled with reticulations. 



Of purely terrestrial forms, Paris quadrifolia and Tamus com- 

 munis have retained the curvilinear ribs, as of Potamogeton natans, 

 and the interstices are filled in with reticulations, as in the last- 

 named plant ; but the inarching process on the margin is becoming 



pronounced, especially in Tamus. 



The Aroidece furnish several varieties. Many are aquatic or 

 subaquatic still; but others are terrestrial. Commencing with 

 Acorus Calamus, we seem to see an aerial form of the ribbon- like 

 submerged leaf of Sigittaria and Vallisneria ; Arum maculatum 

 produces small elliptical leaves at first, recalling tlie form of the 

 floating leaves of Potamogeton natans, or an occasional one of 

 Sagittaria sagittcefolia ; while the sagittate form, which might 

 almost be called typical of the Aroidece, is paralleled with the 

 immature leaves of the Nymphceacece, and the perfect ones of 

 Sagittaria. Bichardia sometimes attains to the peltate-sagittate 



form of Victoria. 



As a very advanced type, Arum tripkyllum may be mentioned. 

 This agrees very closely with exogens, in that it is not only 

 divided into three Yery distinct portions, if they may not be 

 called leaflets, but it has a most perfect degree of the marginal 

 inarching process, and is covered with reticulations. 



Another advanced condition is seen in the leaf of Diefenhachia 

 seguinum. This has a strong midrib and innumerable veins given 

 off iu a pinnate manner, just as described above in the terrestrial 



