light, and only a comparatively small addition may thus be made to the flora of a garden. The conditions described above are such that it has not been found possible to grow in one place more than fifteen thousand species of the higher plants. It will be found, moreover, that a large number of the species included are not able to attain normal stature and appearance, and will thus be useless in representing the form intended.
In consequence of this limitation of the number it is customary to supplement the living plants by collections of prepared specimens of contemporaneous and fossil forms, in order to represent
The Main Palm House of the Royal Gardens at Kew, with Lake in Foreground.
more completely the vegetation of the globe. The living as well as the prepared plants are generally so assembled as to demonstrate the descent and relationship of the different groups, distribution over climatic and geographic zones, as well as their principal biological adaptations to the factors to be met in their native habitats. In addition to this strictly natural method of treatment it is also customary to illustrate by proper groups the forms which have become of special interest because of their food-furnishing, textile-yielding, medicinal, or other economic value. In order to accomplish these purposes a suitably equipped garden must contain, besides the necessary facilities for growing plants, museum buildings arranged for the display of prepared specimens, and if it designs to afford opportunities for research it must also be furnished with a library and laboratory facilities.
There are in the world more than two hundred institutions designed as botanic gardens, a large proportion of which are devoted to the cultivation of decorative plants, or subserve the use
