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256 TROPICAL NATUREof palms or of tree-ferns. Some grow best over water, othersmust be elevated on loftytrees and well exposed to sun andair. The wonderful variety in the form, structure, and colourof the flowers of orchids is well known ;but even our finestcollections give an inadequate idea of the numbers of theseplantsthat exist in the tropics, because a large proportion ofthem have quite inconspicuous flowers and are not worthcultivation. More than thirty years ago the number of knownorchids was estimated by Dr. Lindley at three thousand species,in Bentham and Hooker's Genera Plantarum at five thousand,and it is not improbable that they may be now nearly sixthousand. But whatever may be the numbers of the collectedand described orchids, those that still remain to be discoveredmust be enormous. Unlike ferns, the species have a verylimited range, and it would require the systematic work of agood botanical collector during several years to exhaust anyproductive district say such an island as Java of its orchids.It is not therefore at all improbable that this remarkablegroup may ultimately prove to be the most numerous inspecies of all the families of flowering plants.Although there is a peculiarity of habit that enables onesoon to detect an orchidaceous plant even when not in flower,yet they vary greatly in size and aspect. Some of the smallcreeping species are hardly larger than mosses, while the largerGrammatophyllums of Borneo, which grow in the forks of trees,form a mass of leafy stems ten feet long, and some of theterrestrial species as the American Sobralias grow erect toan equal height. The fleshy aerial roots of most species givethem a very peculiar aspect, as they often grow to a greatlength in the open air, spread over the surface -of rocks, orattach themselves loosely to the bark of trees, extractingnourishment from the rain and from the aqueous vapour ofthe atmosphere. Yet notwithstanding the abundance andvariety of orchids in the equatorial forests, they seldomproduce much effect by their flowers. This is due partly tothe very large proportion of the species having quite inconspicuousflowers ;and partly to the fact that the floweringseason for each kind lasts but a few weeks, while differentspecies flower almost every month in the year. It is alsodue to the manner of growth of orchids, generally in single