THE BEAN FAMILY. 



295. Classification of Seed Plants. Seed plants (see 

 Art. 1) are divided into two main groups Gymnosperins 

 and Angiosperms. In Grymnosperms the ovules (and there- 

 fore the seeds) are not enclosed in an ovary, or seed-vessel ; 

 in most (though not all) cases they are carried on the surface 

 of the flat carpel-leaves. In Angiosperms, on the other hand, 

 there is an ovary or seed-vessel formed of united carpels, or 

 of a single carpel with united margins. 



Angiosperms are divided into two classes a lower class 

 (Monocotyledons) and a higher class (Dicotyledons), the 

 former having an embryo with one cotyledon, the latter an 

 embryo with two cotyledons. Even this distinction is not an 

 absolute one, and the other distinguishing marks are even 

 more liable to exceptions if taken singly, but on the whole 

 it is always easy to tell a Monocotyledon from a Dicotyledon. 



Monocotyledons generally have the stem-bundles scattered 

 in cross-section, the individual bundles are " closed " (without 

 a cambium-layer), the main leaf -bundles (veins) are parallel 

 and connected by delicate cross-veins, and the flower-parts 

 are in threes. In Dicotyledons the stem-bundles are generally 

 arranged in a single ring as seen in cross- section, the bundles 

 are "open" (with cambium between bast and wood), the 

 finer leaf- veins form an irregular network, and the flower- 

 parts are in twos, fours, or fives. Very few plants " break " 

 more than two of these rules, and no plant breaks them all ; 

 in exceptional cases the plant's position is usually easy to 

 define on its general affinities, instead of by applying more 

 or less arbitrary laws. 



296. Characters used in Classification. The general 

 rule with regard to the kind of characters used in the classify- 

 ing of plants is that the less any part of the plant is concerned 

 with special habits the more important is it for classification. 

 For instance, the vegetative organs (roots, stems, leaves) are 

 of little value in characterising the larger groups, because 

 they are on the whole more liable to variation and modifica- 

 tion than are the flowers and fruits, though some vegetative 

 characters (e.g. the alternate or opposite arrangement and the 

 reining of the leaves) are less liable to modification than 

 others and can be used in classification. Of the floral 

 characters, the most generally useful is cohesion (Art. 257), 



