225 

 212. -BOTANICAL NOTES. 



No. 9. — Mechanical action and irritahility in the floicers of " Cafa- 



setum tridentatum" Hoolc. 



Darwin in his work on the " Fertilization of Orchids," carefully 

 describes the structure of the flowers of Gatasetum tridentatiim., Bool:. 

 He states that the labellum stands uppermost, forming a helmet or 

 bucket protecting the column and in a position in which it cannot 

 hold nectar,* while his drawings show the position of the antemiiB 

 or horns of the column and their position. He argues also the 

 hypothesis that the antennae are possessed of a special irritability 

 which leads to the ejection of the pollen. Kerner and Oliver in 

 " Natural History of Plants," follow Darwin, but their illustrations 

 show the flower in a reverse position to that in which Darwin 

 describes it to stand, and it is evident from the text that it has 

 been drawn so intentionally. 



Darwin, however, is quite correct in describing the position ot 

 the flower, and Kerner is as certainly so as Darwin, for the truth is 

 that sometimes the flower stands one way, sometimes the other, and 

 also at every possible angle. Gatasetum tridentatum, HooTc., is one 

 of the commonest of Trinidad orchids, flowering every season in the 

 Botanic Gardens, and numerous observations on its process of fertili- 

 zation have been registered. Kerner as well as Darwin discusses 

 the special irritability of the antennas, and declares that nothing 

 special has been found " to account for it." Darwin allows that in 

 some instances the pollen is ejected by other means, but lays great 

 stress upon the sensitiveness of the antennae. " / find no moderate 

 degree of violence on any part of the flowers, excepting the antennce, 

 produces any efect." It is certain, however, that the ejection of the 

 pollinia can be caused by other means than the irritation of the 

 antennae by touch, for it can be easily proved that a concussion of the 

 flower, the removal of the anther-cap, and pressure exerted on 

 almost any part of the column, and especially any irritation on the 

 margins of the stigmatic pit, will effect this readily if the flower is 

 at a favourable stage of maturity, and irritation of one side will 

 cause the gland of the pollinia to be thrown to the same side, and 

 vice versa ; while if both sides are irritated at the same moment the 

 gland is thrown directly to the front. 



* Page 232. 



