Lastly, in this connexion may be mentioned the very interesting and beautiful group of orchids (_Orchidaceæ_), many of which live high up in the air, supported on the branches of trees, from which their roots hang freely down. Such orchids are sometimes spoken of as "air-plants."
All the Angiosperms as yet mentioned, from the grasses to the orchids inclusively, belong to the lower of the two great groups or classes into which, as was lately said, the whole mass of Angiosperms is divided.
This great group is named _Monocotyledones_ (on account of the structure of the seed), and it is sometimes spoken of as _Endogens_, in reference to a generally prevalent habit of growth. The members of this whole class will then hereinafter be spoken of as "Monocotyledons."
All the plants which yet remain to be enumerated belong to the other and still greater group of Angiosperms called (also in reference to their seeds) _Dicotyledons_, a group sometimes spoken of as "_Exogens_," in reference to the habit of growth prevalent amongst its species.
All our familiar trees which are not conifers, and most of our flowering shrubs and herbs, are "Dicotyledons."
Amongst the many orders which compose the Dicotyledonous group the few following may be selected for enumeration, either on account of the general interest they possess, or because they will have to be more or less referred to hereafter.
We may thus note the singular order of vegetable parasites, the _Loranthaceæ_, an order containing some thirty genera with four hundred species, and including the mistletoe, which is traditionally venerable in our island. The great group of catkin-bearing trees (_Amentaceæ_), contains a great assemblage of plants, familiar in England, such as the hornbeam, hazel, oak, beech, Spanish chestnut, birch, willow, poplar, &c.[24]
