48 



INTRODUCTION. 



The flowering plants are also divisible into two classes, founded on dis- 

 tinct kinds of structure and growth. One the Exogens, increasing from the 

 periphery ; and the other the Endogens, increasing from the interior. The 

 embryo of the first having two cotyledons, it has also been called Dicotyle- 

 donous, and that of the second having but a single one, it has received the 

 name of Monocotyledonous. These characters have already been spoken of, 

 as well as the distinction between them in the venation of their leaves. 



But there is a group of Exogenous plants, in which the ovules and seeds 

 are naked, instead of being contained in an ovary or pericarp ; there is also 

 a group of spurious Exogens, which have the same peculiarity, and the two 

 have therefore been formed into a separate class, under the name of Gymno- 

 gens or Gyrnnospermous (naked-seeded) plants. 



A fourth class, containing a few but most extraordinary plants, appears 

 to form a passage from the Endogens to the Thallogens. They are composed 

 of cellular tissue, and bear flowers, and yet are propagated by spores and not 

 by seeds. This class has received the name of Sporogens or Rhizanthece, 

 (root flowering). 



The six classes of plants may be thus characterized : 



I 1. Exogens or Dicotyledons. Structure of stern 

 exogenous. Seeds in a pericarp. Em- 

 bryo with two cotyledons. Leaves reti- 

 culated. 



2. Gymnogens or Gymnosperms. Seeds naked. 



t-< • u j -.1. t?i Embryo with two or more cotyledons, 



rurnished with b lowers. / -& ? -mt < i j i t c 



3. J^ndogens or Monocotyledons, Structure ot 



stems endogenous. Seeds in a pericarp. 

 Embryo with one cotyledon. Leaves pa- 

 rallel, veined. 



4. Sporogens or Rhizanths. Structure mainly 

 cellular. Pericarp containing spores in- 

 stead of seeds. Embryo none. 



5. Acrogens having a regular stem growing from 

 Destitute of Flowers. ^ the apex, and clothed with leaves. 



6. Thallogens. Stemless, rootless, and leafless. 



These classes are of very different extent, as regards the number of spe- 

 cies contained in them ; the exogens having more than all the other flowering 

 plants together ; the endogens are also very rich in species, as are also both 

 the acrogens and thallogens, whilst the two others possess but few. 



Many divisions or sub-classes have been proposed in these classes, almost 

 all of which are exceedingly artificial, but are convenient for practical pur- 

 poses. Jussieu has split the exogens into three divisions founded on the pre- 

 sence, union, or absence of the petals ; these have been generally adopted : 

 they are : 



Polypetalce. Calyx and corolla both present, the latter having distinct 

 petals. 



Monopetalce. Petals united. 



Apetake. Petals wanting. 



These divisions, as well as the other classes, have been variously subdi- 

 vided by different authors, but it would be a useless task to attempt to notice 



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