Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/286

266 importance to Orchids by attracting insects, which are indispensable for the fertilisation of most of the species, yet good reasons can be assigned for the belief that nectar was aboriginally an excretion for the sake of getting rid of superfluous matter during the chemical changes which go on in the tissues of plants, especially whilst the sun shines. The bracteæ of some Orchids have been observed to secrete nectar, and this cannot be of any use to them for their fertilisation. Fritz Müller informs me that he has seen such secretion from the bracteæ of an Oncidium in its native Brazilian home, as well as from the bracteæ and from the outside of the upper sepal of a Notylia. Mr. Rodgers has observed a similar and copious secretion from the base of the flower-peduncles of Vanilla. The column of Acropera and Gongora likewise secretes nectar, as previously stated, but only after the flowers have been impregnated, and when such secretion could be of no use by attracting insects. It is in perfect accordance with the scheme of nature, as worked out by natural selection, that matter excreted to free the system from superfluous or injurious substances should be utilised for highly useful purposes. To give an example in strong contrast with our present subject, the larvæ of certain beetles (Cassidæ, &c.) use their own excrement to make an umbrella-like protection for their tender bodies.

It may be remembered that evidence was given in the first chapter proving that nectar is never found within the spur-like nectaries of several species of Orchis, but that various kinds of insects penetrate