238 EXOGENS. 



collects Peppers, Nyctaginacese, Amaranths, and Chenopods, by the common 

 character of their stems having several sets of fibrovascular bundles, as in 

 Endogens. It is however evident, as is stated in the proper places of 

 this work, that the character proposed by Schleiden is no more universal 

 than that which has been mentioned by myself; and therefore I think it more 

 prudent to defer for the present an attempt at maintaining the Class of Homo- 

 gens, and to leave it to be determined by future and very extended inquiries, 

 whether such a group really exists in nature, what are its limits, and how 

 they are to be defined. It seems probable that some such a group does 

 exist, or at least that in the stems of Dicotyledonous plants there are modifi- 

 cations of structure of the very highest importance, to which attention has 

 been hitherto insufficiently directed. 



If Exogens are distinctly known from Endogens by their peculiar manner 

 of growtlf and by the arrangement of their woody matter, they are not less 

 clearly defined by external marks. 



Their leaves have the veins ramifying from the midrib, or ribs if there are 

 several, in so intricate a manner as to give the appearance of irregular net- 

 work. Their veins never run parallel with each other without ramifications; 

 for if, as sometimes happens, they appear to do so, it will be found that the 

 appearance is confined to the principal veins or ribs, and that the secondary 

 veins between them ramify in the usual way. The leaves are moreover in 

 most cases articulated with the stem, leaving behind them a clean scar when 

 they die, not rotting away and hanging upon the stem in the form of a 

 ragged sheath, as is common in Endogens. Moreover, they are frequently 

 furnished with stipules, an unusual circumstance in Endogens. 



The flowers of Exogens are usually constructed upon a quinary type, that 

 is to say, have five sepals, five petals, and five stamens, or some power of that 

 number ; now and then they vary to a type of four, or they exceed the 

 number five ; but we rarely find the ternary structure of Endogens present 

 in them. If, as in Crowfoots, Berberids, Anonads, and other Orders, the 

 sepals and petals follow a ternary type, the number three is lost in the sta- 

 mens or the ovary. The Natural Order of Menispermads is the only one 

 among Exogens in Avhich the ternary type regularly pervades all the parts 

 of the flower. 



In their manner of growth they rarely resemble Endogens. The conse- 

 quence of the ramification of the veins is to give their leaves a broad and 

 rounded figure, the effect of which upon their general appearance is to pro- 

 duce the round-headed aspect that we recognise in all the trees naturally 

 inhabiting this country. In no known instance does the stem grow by the 

 development of a single terminal bud ; so that we never find in this class 

 the columnar aspect of Palm-trees, unless the genus Theophrasta be con- 

 sidered an exception. 



The differences between Exogens and Endogens, thus strongly marked in 

 the stem, leaves, and flowers, are connected with others in the embryo. 

 In Exogens of the common kind this organ has two lobes, held together by 

 a minute central body, the upper end of which, between the lobes, is the 

 plumule or rudimentary stem, the lower the radicle or rudimentary root ; 

 the lobes themselves, or cotyledons, are rudimentary leaves. This structure 

 is readily seen in a hazel-nut or a garden-bean ; the deviations from it are 

 few and unimportant as compared with those of Endogens. Three or a 

 greater number of cotyledons may be present in a whorl, instead of two 

 opposite to each other. Or one of the two cotyledons may be much smaller 

 than the other, as in Trapa ; or they maybe deeply lobed, as in the garden- 



