In greenhouses where plants requiring very different optimum temperatures and illumination are kept together.
As regards water, its deficiency or excess is a relative matter, and although many of the minor maladies of pot-plants in windows and greenhouses controlled by amateurs depend on its misuse, water alone is probably never a primary cause of disease.
In greenhouses where the suns rays are concentrated on particular spotsand a certain class of obscure diseases, such as silver-leaf in plums, foxy leaves in various plants, may also be placed here.
Travellers are especially struck with the beauty of some of the wild flowers, more especially with the lilies and convolvuluses; and European greenhouses have been enriched by several Formosan orchids and other ornamental plants.
The most familiar species is P. senilis, a Mexican plant, which though seldom seen more than a foot or two in height in greenhouses, reaches from 20 ft.