292 ANNUAL OF SCIENTIFIC DISCOVERY. 



and base of the labellum is excellently well adapted to favor an insect 

 striking the crest when it raises its head, after having crawled up the 

 labellum, and licked up the last drop of nectar at its base. The crest 

 of the rostellum is so exquisitely sensitive that a touch from a most 

 minute insect causes it to rupture at two points, and instantaneously 

 two drops of viscid liquid are expelled, which coalesce. This viscid 

 fluid sets hard in so wonderfully rapid a manner that it rarely fails to 

 cement the tips of the pollinia, nicely laid on the crest of the rostel- 

 lum, to the insect's forehead. The pollen-masses, when once cemented 

 to an insect's forehead, will generally remain, firmly attached to it 

 until the viscid stigma of a mature flower removes these encum- 

 brances from the insect, by rupturing the weak elastic threads by 

 which the grains are tied together, receiving at the same time th 

 benefit of fertilization." 



Mr. Darwin also describes the curiously constructed arrangements 

 for reproduction existing in the male flowers of another variety of 

 orchids, viz., the Catasetum. It is necessary that the pollen-masses of 

 these flowers be transported to female plants in order that seed may 

 be produced. Now, in these male flowers, the pollinium is furnished 

 with a viscid disc of huge size ; but the disc, instead of being placed, 

 as in other orchids, in a position likely to touch and adhere to an 

 insect visiting the flower, is turned upwards and lies close to the 

 upper and back surface of a chamber, which must be called the stig- 

 matic chamber, though functionless as a stigma. There is nothing in 

 this chamber to attract insects ; and even if they did enter it, it is 

 hardly possible that the disc should adhere to them, for its viscid sur- 

 face lies in contact with the roof of the chamber. How then does 

 nature act ? She has endowed these plants with what must be called, 

 for want of a better term, sensitiveness, and with the remarkable 

 power of forcibly ejecting their pollinia to a distance. Hence, when 

 certain definite points of the flower are touched by an insect, the 

 pollinia are shot out like an arrow which is not barbed, but has a 

 blunt and excessively adhesive point. The insect, disturbed by so 

 sharp a blow, or after having eaten its fill, flies sooner or later to a 

 female plant, and, whilst standing in the same position as it did when 

 struck, the pollen-bearing end of the arrow is inserted into the stig- 

 matic cavity, and a mass of pollen is left on its viscid surface. Thus, 

 and thus alone, species of the genus Catasetum are fertilized." 



Notwithstanding the immense seed-produce of orchids, the greatest 

 care is taken throughout this vast order, with its more than four hun- 

 dred genera and its six thousand species, that the pollen shall not be 

 wasted ; and yet, so far as all observation goes, the act of fertilization 

 is for the most part left to insects. In commenting on this curious 

 circumstance, Mr. Darwin uses the following language : " Consider- 

 ing how precious the pollen of orchids evidently is, and what care has 

 been bestowed on its organization and on the accessory parts, con- 

 sidering that the anther always stands close behind or above the 

 stigma, self-fertilization would have been an incomparably safer pro- 

 cess than the transportal of the pollen from flower to flower. It is an 

 astonishing fact that self-fertilization should not have been an habit- 

 ual occurrence. It apparently demonstrates to us that there must 

 be something injurious in the process. Nature thus tells us, in the 

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