Ch. XIII] THE DICOTYLEDONS 539 



specialized in the extreme, having inferior ovary, 1 or 2 

 stamens consolidated with the style near the stigma, zygo- 

 morphic perianth, and other elaborations of detail which have 

 a meaning, or at least a degree of specialization, far beyond 

 explanation through utility alone (Fig. 383). Many are 

 wonderfully adapted to the visits of particular insects, even 

 to single species. The seeds are mostly so minute that they 

 float with the dust in the air, and thus achieve wide dispersal. 

 Hardly any economic forms occur in the order, aside from 

 Vanilla, which is now supplanted by the synthetic substance. 

 They yield, however, the most treasured plants of our exotic 

 greenhouses, and are the special favorites of wealthy lovers 

 of plants. Some kinds, Cattleya, Lselia, Dendrobium, 

 Oncidium, may be bought from florists, while Cypripe- 

 dium, Orchis, Arethusa, Pogonia, Habenaria, Calopogon, 

 Calypso, are rare and exquisite forms of our own flora. 



Sub-class 2. Dicotyledone^ : the Dicotyledons, or 

 Exogens 



These include most of the Angiosperms, — some 108,000 

 out of the 133,000 species of that group — with the highest, 

 largest, and dominant families. They differ from the Mono- 

 cotyledons in that the embryo has two cotyledons instead 

 of one, the leaves are almost invariably netted-veined, the 

 stem is exogenous with clear distinction of bark, wood, and 

 pith and with indefinite growth by aid of a cambium, the root 

 system is mostly derived, as in Gymnosperms, from a 

 branching primary root, and the flowers have typically five 

 parts to a whorl, with sometimes four and very rarely three, 

 — the petals and sepals being usually unlike. The fruits 

 are diverse, and adapted to various modes of dissemination. 

 While the lower forms are both pollinated and disseminated 

 by wind, the higher make use of insects and other animals 

 in both functions, often displaying notable adaptations to 

 that end. Further, as in Monocotyledons, both flowers and 

 fruits exhibit in the higher forms a structural specialization 



