EXOGENS. 239 



cress. But in all these cases the dc\datioiis arc obviously reeoncileablc with 

 the t}']3ical 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. 



Hence the class of Exogens has five important, and, in some measure, 

 independent characters, b}'- which its Hmits are settled. 



1. The wood is exogenous. 



2. The veins of the leaves are netted. 



3. The fructification is formed upon a quinary or quaternary type. 



4. The embryo is dicotyledonous. 



5. The germination is exorhizal. 



Exogens have received other appellations in allusion to such characters ; 

 they are commonly called Dicotyledones, and Exorhizse is another but less 

 common appellation ; moreover, they are the Phanerocotyledone?e of 

 Agardh, the Anthophytse and Carpophytse of Oken's school, the Dichor- 

 gana of Schidtz, the Phylloblastaj of Reichenbach ; not to mention names 

 still more obscure. 



In consequence of imperfect development, and the abortion or midtiph- 

 cation of parts, many deviations occur from the above characters. But, as 

 in Endogens, so in these, such anomalies do not cause any real difficulty in 

 distinguishing Exogens from other plants. Suppose the stem to be so 

 slightly formed, as in Mossweeds (Podostemacea^), or the aquatic Hippm-ids, 

 as not to arrive at a state in which the exogenous aiTangement is percepti- 

 ble, we have the dicotyledonous embryo, and the typical number of the 

 floral organs to guide us. Let the leaves appear as scales, as in Lathrfea, 

 Orobanche, and the like ; still there is the embryo or again the floral 

 proportions. If the fructification is absolutely ternary as in Menispermads, 

 the organisation 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 om' faUing into eiTor. It is therefore always to be remembered, 

 that the hmits 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 undiscoverable. 



But while the class of Exogens is thus distinctly circumscribed, it is 

 found to approach the limits of other classes at various points. It evidently 

 touches Gymnogens by means of Beefwoods (Casuarinace^e) ; Endogens 

 are represented by Crowfoots, some of the species of Ranunculus having 

 a striking resemblance to Alismads, and perhaps by Peppers, which seem 

 to have a tendency to Arads. Menispermads may almost be mistaken for 

 Sarsaparillas (Smilacea)), and thus a connection is established with Dic- 

 tyogens ; Mossweeds (Podostemacese) may be regarded as analogous to 

 Liverworts among Thallogens ; it is not unreasonable to regard Hippurids 

 as an exogenous form of Arrowgrasses, or Callitriche as the analogue of 

 Lemna, and the whole Nymphal Alliance certainly comes very near to 

 Hydrocharads. 



The different methods of classifying Exogens have been considered in 

 the introductory part of this work. That which is here adopted is founded 

 on the following considerations. 



The office of reproduction is, after that of sustaining life, the most 

 essential in the economy of both plants and animals, and therefore the 



