104 



ANALYTICAL CLASS-BOOK OF BOTANY. 



All growth takes place between the bark and wood, by 

 the addition of layers to each. In herbaceous species, 

 though the wood is absent, the distinction between the 

 bark and pith still remains. The leaves, when present, 

 are distinguished by their branching veins, and are hence 

 called net, or feather-veined leaves. Their seeds have 

 always two opposite cotyledons, except in the Pine 

 family, which have several. The parts of their flowers 

 are not usually multiples of three, but of some other 

 number. When they have two series of floral envelopes, 

 the calyx and corolla are usually distinguished by differ- 

 ences of color and appearance. 



Endogens, on the contrary, possess a stem not distin- 

 guishable into bark, wood, and pith, but their structure 

 is homogeneous. The woody stems of this class are 

 composed of bundles of woody fibre disposed without 

 order. The stem is surrounded by an integument which 

 does not increase by layers, like bark. All additions to 

 the woody structure take place in the inside. The her- 

 baceous stems of this class consist chiefly of cellular tis- 

 sue, surrounded by a mere cuticle. The leaves also 

 usually present a remarkable difi'erence from those of 

 Exogens in their parallel veins, which run the whole 

 length of the leaf unbroken. A few Endogens, however, 

 have net-veined leaves. The flowers usually have their 

 parts in threes, or multiples of three. The floral enve- 

 lopes also are rarely distinguishable into calyx and corolla. 

 The seeds are distinguished by having but one cotyledon, 

 or sometimes two, which are alternate. 



Such are the principal technical distinctions between 

 these two classes ; but even the herbaceous species pre- 

 sent a remarkable difference in their general appearance, 

 which will soon become a sufficient guide to the student, 

 in nearly all cases. The Lily and Orchis tribes, and the 

 Grasses, are our best examples of Endogens, as the 

 Palms, and other arborescent species, are confined to 

 tropical countries. All our forest trees, our shrubs, and 

 the great majority of our most common herbaceous 

 plants, such as the Pea and Bean tribe, and the Umbel- 

 liferous family, are examples of Exogens. 



Exogens are again divided into two sub -classes 

 Angiospekms {covered seeds), and Gymnosperms {naked 

 seeds), on account of some important differences in their 

 reproductive organs, which, in the latter, make a distant 

 approach to the simplicity of the Ferns and Club-mosses. 

 In these plants the staminate flowers are reduced to 

 mere clusters of stamens, while the ovules, instead of 

 being included in a regular ovary, are borne on the open 

 surface or margin of a carpellary leaf, which takes the 

 place of a pistil, and in the Pine family becomes a har- 



dened scale. This sub-class includes only the Cone- 

 bearing trees, and a small tropical family. The other 

 division, Angiosperms, which have regular pistils with 

 the ovules contained in a proper ovary, comprehends the 

 rest of the Exogens. 



This sub-class which still includes the great majority 

 of flowering plants, on account of its size, is broken up, 

 for the sake of greater convenience, into three nearly arti- 

 ficial sections. 



Those of polypetalous corollas, form the section 

 PoLYPBTALiE ; those with monopetalous corollas are in- 

 cluded under MoNOPETALiE, while those destitute of a 

 corolla constitute the section Apbtal.e. As these, how- 

 ever, are artificial distinctions, there are in each section 

 plants, which, so far as the corolla is concerned, would 

 belong to other sections, but whose affinities compel us 

 to place them where they are. 



Having now considered the principal divisions of 

 Plowering plants, those of Elowerless plants next claim 

 our attention. Cryptogamous plants are divided into 

 three great classes — Acrogens, Anophytes, and Thallo- 

 phytes, depending upon distinctions in the mode of 

 growth. 



The Acrogens are those which have a regular axis 

 of growth, like flowering plants. They have also the dis- 

 tinctions of stem, root, and foliage. Their stems consist 

 of woody fibre and ducts, mingled with cellular tissue. 

 In their general structure, they are not much inferior to 

 many Phsenogamous plants, except in their reproductive 

 organs. Their sporangia, or spore-cases, and their 

 spores, bear but a very distant resemblance to the flowers 

 and seeds of Flowering plants. They are called Acro- 

 gens {plants growing at top), because the axis grows 

 only at the summit, and never increases in diameter. To 

 this class belong the Scouring Rushes, Club-mosses, and 

 Ferns. 



The Anophytes {plants growing upwards), gene- 

 rally preserve the distinctions of root, stem, and leaves ; 

 and their reproductive apparatus still bears a remote re- 

 semblance to that of flowering plants. The stem grows 

 upwards, and the root downwards ; but their whole 

 structure is composed solely of cellular tissue, except in 

 a very few species which have ducts, and hence they are 

 restricted to very low and humble forms, never exceeding 

 a few inches in height. The Mosses and Liverworts are 

 the chief examples of this class. 



But when we reach Thallophytes, all distinction 

 between root, stem, and leaves, ceases ; there is no longer 

 any axis of growth, nor are there any organs correspond- 

 ing to the reproductive apparatus of higher types. Theso 



