172 THE NATURAL SYSTEM. 



■with leaves, and composed of cellular tissue and scalariform ducts (Ferns, 

 mosses, club-mosses, horsetails, etc.) 

 ThallogeNjE (duA/.or, green branch, ytwuu). 



Flowerless plants producing in vegetation a thallus, with no distinction 



of stem and leaf, or root, and composed of cellular tissue only (Lichens, 



fungi, etc.) 



900. Classes of the flowerless plants. For the sake of analogy 



and an advantageous comparison with the Phaenogams, we may also 



regard these two provinces of the Cryptogams in the light of Classes 



founded upon their different modes of fruitbearing. Thus the Acro- 



gens constitute the class 



AKGIOSI'ORiE (ayyeioc, oiropu). 



Acrogenous plants, producing their spores in sporangia (vessels) which 

 burst when the spores are mature. 

 GyhnOSPOR^E (yvfivbc, CTTopu). 



Thallogenous plants reproduced by spores, which aro produced in parent 

 cells, either forming a part of the vegetating thallus, or growing upon the 

 surface of some definite region of the thallus. 



901. Fourth division — cohorts. The six classes, as above constituted, are- 

 still each of great extent, — too great for the comprehension of the learner, or prac- 

 tical use. A further division is therefore necessary. To effect this on strictly 

 natural principles botanists have labored hitherto in vain. The truth is, the 

 members of these several classes are united by affinities so equable as to render 

 it impossible to subdivide, except by distinctions more or less arbitrary. So 

 adjacent territories, where rivers or other natural boundaries are wanting, must bs> 

 separated by artificial lines. 



902. The axgiosperjis are divided by De Candolle, in his great descriptive 

 work " Prodromus Systematis Naturalis Begni Vegetabilis" into four sub-classe3 

 founded upou the conditions of the floral envelops, viz. : 



1. Tkalamiflorce, petals distinct, and (with the stamens) hypogynous. 



2. Calyciflora, petals (with the stamens) perigynous. 



3. CoroUiflorce, petals united, hypogynous, stamens epipetalous. 



4. Monochlamydece, petals wanting. 



903. The plan of Endlicher in his "Genera Plantarum" is more 

 simple and convenient, and has been followed by American writers 

 generally. He separates the Angiospermse into three " cohorts," thus : 



1. Dialypetalce (SiaTiuu, to dissolve). Exogenous plants, having both calyx 

 and corolla, the latter composed of distinct petals (polypetalous), sometimes 

 slightly cohering by the base of the stamens, rarely abortive. 



2. Gamopetaloe (yaixor, union). Exogenous plants, having both calyx and 

 corolla, the latter composed of petals more or less united. 



3. Apetalce (a, privative). Exogenous plants with flowers having a calyx 

 only, or neither calyx nor corolla (achlamydeous). 



904. The class petalifer^e may be conveniently separated into 

 two cohorts, as follows. 



1. Spadiciflorce. Endogenous plants with flowers having no perianth, or a 

 scaly one, and borne on a thickened spadix, which is often enveloped in a 

 spathe. 



