EXOGENS. 



EXOGENS. 



654 



appearance is to produce the rounded lumpish 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 

 [GYJCJOSFERMS], unless the genus Tlieophrasta be considered an excep- 

 tion. Consequently a landscape consisting of nothing but Exogenous 

 plants would resemble the imaginary scene that forms the subject of 

 the preceding cut. 



The differences between Exogens and Endogens, thus strongly 

 marked in the stem, leaves, and flowers, are connected with others in 

 the embryo. [REPRODUCTION IN PLANTS ; SEED.] 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 may be deeply lobed, aa in the garden-cress. But in 

 all these cases the deviations are obviously reconcileable with the 

 typical character of being Dicotyledonous. 



When the embryo of an Exogen germinates, the radicle simply 

 lengthens at its point, without having to break through the coat of 

 the embryo ; on this account Exogens have been named Exorhizal. 



The result of this examination is, that the great class of Exogens 

 has five important, and in some measure independent characters, by 

 which its limits are settled. 



1. The wood is exogenous. 



2. The veins of the leaves are netted. 



8. The fructification is formed upon a quinary or quaternary 

 type. 



4. The embryo is dicotyledonous. 



5. The germination is exorhizal. 



Hence Exogens have received two other names in allusion to such 

 characters ; they are commonly called Dicotyledona ; and JSxorhizce 

 is another but less common appellation. Moreover, they are the 

 Phanerocotyltdonea; of Agardh, the Anlhophytce and Carpophytte of 

 Oken's school, the Dichorgana, of Schultz, the Phylldbltaloe of 

 Reichenbach ; not to mention other names still more obscure. 



In consequence of imperfect development, and the abortion or 

 multiplication of parts, many deviations occur from the above 

 characters. But as in Endogens, so in these, there is not in con- 

 sequence any real difficulty in distinguishing Exogens from other 

 plants. Suppose the stem to be so slightly formed, as in Podostemaceis 

 or the aquatic 1/aloragea, as not to arrive at a state in which the 

 exogenous arrangement is perceptible, we have the dicotyledonous 

 embryo and the typical number of the floral organs to guide us. 

 Let the leaves appear as scales, aa in Lathrrea, Orobanche, and the 

 like ; still there is the embryo or again the floral proportions. If the 

 fructification is absolutely ternary as in Meniipermacece, the organi- 

 zation of the stem, leaves, and embryo reveals the true nature of 

 such plants. Or if the embryo is undivided, as in Cuscuta, and at 

 the same time the veins of the leaves deficient, and all this with an 

 incomplete formation of woody matter, then the number of parts in 

 the flower remains to prevent our falling into error. It is therefore 

 always to be remembered, that the limits of this great class are not 

 exclusively determined by one single character, but by a combination 

 of five; a part of which may be occasionally exceptional or undis- 

 coverable. 



Like all other natural assemblages, Exogens have many analogies 

 with other parts of the vegetable kingdom. We have already 

 adverted to the Podostemaceous order of this class representing 

 distinctly the Pistiaceous order, or at least Lemna among Endogens. 

 Piperacece are distinct analogies here to the Aracece of Endogens, 

 Ckmopodialet to Glumota, and possibly Mmiipermacete to Smilacete. 



Whatever uses there may be in the vegetable kingdom are to be 

 found in this class, which comprehends four-fifths of the natural 

 orders, and probably not much less than the same proportion of 

 species. Timber, in particular, is their exclusive produce, and if corn 

 has no direct analogy in Exogens, at least a substitute for it is 

 furnished by the potato and the cassava. 



Considering the very . great numbers of Exogens they may be 

 rated at 50,000 or 60,000 at a low computation it is not sin-prising 

 that it should be here that the systematic botanist experiences his 

 great difficulties. Comparatively no embarrassment worth notice 

 occurs in the arrangement of Endogens ; but in Exogens the difficulties 

 are so great as to have hitherto bafiied the most acute writers. We 

 do not mean with regard to the natural orders themselves, for they 

 are in general well understood and defined : our observation applies 

 to a collocation of the orders, or in other words, to the construction 

 of groups of a secondary value which shall be as natural and as well 

 defined as the orders themselves. In a recent enumeration we find 

 no fewer than 231 orders of Exogens. It is obviously impracticable 

 to study so large a number of combinations without breaking 

 them into groups, and accordingly various methods have been 

 proposed. 



Jussieu, adopting to a certain extent the views of his predecessors, 

 considered 1st, the separation of the petals, 2nd, their combination, 

 or 3rd, their absence, of primary consequence ; and adding to this, 

 4th, the separation of the sexes in flowers having no petals, he 

 formed the four groups of 1st, Polypetalous ; 2nd, Monopetalous ; 

 3rd, Apetalous; and 4th, Diclinous plants. The first three of these 

 he again subdivided according as their stamens or their corolla grew 

 under the ovary (hypogyuous), upon the calyx (perigynous), or upon 

 the ovary (epigynous) ; then the monopetalous epigynous group was 

 subdivided into plants having united stamens and those having them 

 distinct ; the result being eleven classes, which were placed by Juasieu 

 in the following order : 



[" Stamens epigynous 



Apetalous < Stamens perigynous . 



(_ Stamens hypogynous 



I Corolla hypogynous . 

 Corolla perigynous 

 I" anthers united . 

 Corolla epigynous < 

 I anthers distinct 

 I" Stamens epigynous ... 

 Polypetalous < Stamens hypogynous . . . 



L Stamens perigynous . 

 Diclinous 



Class. 

 1 

 2 

 3 

 4 

 5 

 6 



7 



8 



9 



10 

 11 



This was however so artificial a distribution, that botanists soon 

 found it as unsatisfactory as it was simple. Various changes have 

 therefore been recommended from time to time, some of which are 

 the following : 



In 1813, De Candolle, dropping the names of all Jussieu's classes, 

 and abolishing many of them, proposed to arrange as follows the 113 

 orders of Exogens with which he was at that time acquainted. 



Class. 



Polypetalous / Petals hypogynous (TAatoft^o,^) . 1 



[ Petals perigynous (ualycijtone) . . 2 



Apetalous .... (MonoMamydece) . 5 



Thus the classes were reduced from eleven to five, which was a 

 defect ; but those which remained were supposed to be more natural, 

 which would have been an advantage. Five years afterwards, in his 

 ' Regni Vegetabilis Systema Naturale,' he added the names inclosed 

 within parentheses, and he broke up the Thalamiflorce into five cohorts, 

 but without stating what orders he arranged under them. We do 

 not find that he ever pursued the sxibject farther. Since that period 

 this great botanist has occupied himself with the special study of 

 the natural orders, and the public has derived no advantage from his 

 general views, which is much to be regretted. 



In 1825, Professor Agardh of Lund, afterwards bishop of Carlsbad, 

 proposed a great change in the subordination of Exogens, retaining 

 the principles of primary division recognised by Jussieu and De 

 Candolle, but forming them into twenty subdivisions, defined by 

 various characters analogous to those by which the orders themselves 

 are circumscribed. This, we believe, is the first step of any con- 

 sequence towards putting Exogens into a more natural grouping than 

 that of Jussieu. In many respects the subdivisions are, as far as 

 they go, unobjectionable ; but they have excited scarcely any attention 

 among systematic botanists. The necessity however of some better 

 method of subordination than that of Jussieu and De Candolle has 

 become evident to everybody; and attempts have been made to effect 

 this by Drs. Bartling, Schultz, Von Martius, and others on the 

 continent, and by Dr. Lindley in this country. The last named 

 author has successively developed his system in his ' Nixus 

 Plantarum," the ' Penny Cyclopaedia,' article Exogens, his ' Natural 

 System of Botany,' and his ' Vegetable Kingdom.' We shall here 

 give an analysis of Dr. Lindley's arrangement of Exogens, as given 

 in the last mentioned work. Whatever may be its defects they are 

 rather inherent to the subject than attributable to any want of 

 knowledge or judgment in the author, as it undoubtedly displays the 

 largest acquaintance with the details of the structure of this class of 

 plants that has yet been displayed by any writer on systematic 

 botany. The following is an analysis of the class of Exogena : 



Alliances of Exogeus. 



Sab-Class I. Diclinous Exogens. 



Flowers $ ?, without any customary tendency to $5. 



I. AMKNTALF.S. Flowers in catkins, achlamydeous or monochla- 

 mydeous. Carpels superior. Embryo small, with little or no 

 albumen. 



Casnannacere. Ovules 1 or 2, ascending. Radicle superior. 

 Betitlacea. Ovule 1, pendulous. Radicle superior. 

 Al/inyiacea: Ovules 00. Seeds 'winged. 

 Salicacere. Ovules 00. Seeds cottony. 

 Myricacea. Ovule 1, erect. Radicle superior. 



x. Ovule 1, ascending. Radicle inferior. 



