ACROGENS (from &spar, the topmost, and -revvrixo, to produce), in Botany, one of the primary classes of the Vegetable Kingdom, according to the Natural System. This class, with identical limits, is also known by the following designations :— A cotyledons (Jussicu), so named from the fact of the absence of cotyledons amongst this class of plants.
Keenrbryonatic (Richard), designating the absence of any regular embryo in'the reproductive cells, or spores.
Cellidores (De Candolle), denoting the general absence of vascular tissue and the prevalence of cellular tissue in these plants.
Aflame, of various authors, implying the absence of the necessity of the union of two cells in order to reproduce a new individual.
Cryptogamia (Linnreus), intended to convey the idea that if two cells were necessary to the reproduction of the new plant in this class, it was not obvious, as in the Phrenogamous pintas.
The Acrogens, as equivalent to the above expressions for the same class, embrace all those plantri which are included in the above defi nitions. This term itself is, however, only applicable literally to those plants which, destitute of flowers, possess a stem growing in a manner distinctive from those called Ezogens [ExooENs] and Endogens [ENnoosssi. it has been thus restricted by Limll?..t• in his Vegetable Kingdom,' and he places the stemless flowerless plants in another class, called Thallogens. The structure of the stem, however, is at best an artificial character, and the adoption of the terms for the classes expressive of its characters, has rather been accidental than necessary. On this account it is perhaps better to regard the Thallogens as a section of Acrogrns than an independent and equal group.
The stems of Acroyens differ much in appearance from those of Exogens and Endogens The wood is not secreted from layers of tissue, which have the power of reproducing regular zones of wood, as in Exogens, or a regular arrangement of vascular and cellular tissue, as in Endogens. There is generally but a single ring of vascular
bundles even in the Ferns. These vascular bundles do not go on increasing in size, but are all developed together. The lower part of the stem does not continue to increase in size, and its growth is deter mined by the development of new vascular bundles in connection with the leaves of the point or upper part of the stem hence their name. In the Liverworts and Mosses there is ouly a simple vascular bundle in the centre of the stem. In Isoetes too there is a ring of vascular bundles. Science is very deficient in good observations on the structure of the stems of these plants.
The reproduction of the Acrogens is not so simple as was at one time believed. The recent researches of Suminki, Hoffmeister, and others, show that in the Ferns and many other Acrogens the spore is but a transitionary condition, which results in the production of two sets of cells, by the union of which alone can a new individual be produced. [Aura, ItErRODUCTION INJ The following is an analysis of the alliances and orders found in the chums of Acrogens, according to the system adopted by Lindley in his Vegetable Kingdom. Thus Asexual (I) or flowerless plants, A enoorss.
Stem and leaves distinguishable, TUVE Acooctrxss.
Stem and leaves iodintiligniNhHble. THALLOGENS.
Alliances of TUALLOGENB.
Igales.—Cellular flowerless plants, nourished through their whole surface by the medium in which they vegetate ; living in water, or very damp places ; propagated by soospores, coloured spores, or tetraspores.
Peva/ea.—Cellular flowerless plants, nourished through their thallus (spawn or mycelium); living in air; propagated by spores colourless or brown, and sometimes enclosed in nsci ; destitute of green gonidia.
Liebenales.—Cellular flowerless; plants, nourished through their whole surface by the medium in which they vegetate ; living in air ; propagated by spores usually enclosed in asci, and always having green gonidin in their thallus.