November 1, 1899.] 



KNOWLEDGE 



259 



The story of Ice. By W. A. Brend. (Xemies.) Is. 



Our Secret Friends and Foes. Prof. Percy faraday Frankland. 

 Fourth Edition. (S.P.C.K.) Illustrated. 33. 



The Latimer Collection of Antiquities from Porto Rico. By Otis 

 T. ilsson. (Smithsonian Pubhcations.) 



Report of the Chief of the Weather Bureau, 1S97—S. (U. S. 

 Department of Agriculture.) 



The Studio. October. Is. (15, Henrietta Street, Coreut GKirden.) 



THE STORY OF THE ORCHIDS.-III. 



By the Kev. Alex. S. Wilson, m.a., b.sl 



THE most remarkable example of sensitivity among 

 orchids is, however, Catasetum tri'lijiitatmn. The 

 flower is polymorphic, and the three forms were 

 for long referred to distinct genera. All three 

 sorts of flowers are occasionally produced on a 

 single plant. Monacanthua viridis is the female, Mijanthes 

 harhata the hermaphrodite, and Catasttum tridentatum the 

 male variety ; all three being merely sexual forms of the 

 same flower. The rostellum develops two long slender 

 prongs which protrude from the flower. These antennfp 

 are exceedingly sensitive, and if either of them be touched 

 the impulse is transmitted to the membrane of the 

 rostellum, which covers the glandular disk of the 

 pollinium. This membrane suddenly splits, and the 

 poUinium, which the membrane kept curved in a state of 

 extreme tension, springs up, and tearing itself out of the 

 anther-lobe is projected to a distance of several feet like a 

 atone from a catapult. The pollinium is thrown out in 

 the direction from which the stimulus comes, the sticky 

 disk being foremost ; if the cause of the irritation be a 

 bird or large insect the projectile can hardly miss its 

 mark. The ejected poUinia of Catasetum have been found 

 sticking on panes of glass in a greenhouse at a distance of 

 several feet from the flowers. 



Another extra- 

 ordinary flower 

 is that of lyjry- 

 anthes sjttciosa, of 

 Trinidad. The 

 labeUum is in the 

 form of a rounded 

 pitcheror bucket, 

 into which drops 

 of sweetened 

 water continu- 

 ally fall from two 

 hom-Uke projec- 

 tions on the 

 column : over an 

 ounce is collected 

 in this way. 

 When the bucket is full the liquid overflows by a spout at 

 the back. Over the bucket there is an expansion of the 

 labellum (hypochilium), on which certain humble bees con- 

 gregate ; and as they struggle for room, one after another 

 is pushed over into the bucket beneath. With their wings 

 drenched they are unable to fly, and so crawl out by way 

 of the spout in a regular procession. The floral arrange- 

 ments being similar to those in Cypripedium the stigma 

 is pollinated, and the pollen removed as the visitors emerge 

 from their involuntary bath. 



The lie plus ultra of adaptation to insects is, however, 

 attained by Aiu/ranttm sesi/nipe'lah-, an orchid which grows 

 in the island of Madagascar. Its pale white blossoms 

 have a whip-like nectary over a foot in length. Naturalists 

 confidently predicted that an insect with a proboscis of 

 corresponding length would be discovered, and this was 

 ultimately done by Mr. W. A. Forbes, who visited Mada- 



FiG. 8. — Catasetum : in Front and Section. 



Fi&. 9. — Coryanthes 

 speciosa. L. Labellum ; 

 H. Secreting Horns; c. 

 Column. 



gascar and secured a huge sphinx moth having a tongue 

 which measured eleven inches. More recently, Mr. G. F. 

 Scott-Elliot found sphinx moths with proboscides eighteen 

 inches long. This observer also saw sun-birds visiting 

 the flowers of Angraecum. In Brazil, Fritz MiiUer 

 caught a hawk moth with a proboscis of eleven inches ; 

 analogy would lead one to infer the 

 existence of flowers to correspond. 

 The length of the nectary varies a 

 good deal in different specimens of 

 Angraecum, but those flowers in 

 which it is shorter than the visitor's 

 proboscis are placed at a disadvan- 

 tage, for the insect can remove their 

 nectar without bringing its head in 

 contact with the rostellum. Natural 

 selection must therefore tend to 

 perpetuate those flowers with the 

 longest nectaries. The forcible ejec- 

 tion of the pollinia in Catasetum and 

 Dendrobium, already considered, is 

 but another method of securing the 

 same end that is served by the 

 elongated spur of Angraecum. 

 Even among British orchids there is a distinct tendency 

 towards adaptation to special insects. This appears even 

 from a general survey, but is much more obvious when the 

 details of their structure are studied. H. Miiller has 

 remarked that many species which in the lowlands are 

 specialized in relation to bees, have in the Alps acquired 

 deeper nectaries and become adapted to butterflies, which 

 are more numerous than bees at lofty elevations. 



To account for these 

 mutual relationships 

 between orchids and insects 

 would be a comparatively 

 simple matter were we at 

 liberty to make two assump- 

 tions — viz., the existence of 

 highly specialized fertilizing 

 agents to begin with ; and the 

 superiority of cross-fertiliza- 

 tion over self-fertUization. 

 The botanist, in seeking to 

 explain the development of 

 flowers, naturally postulates 

 the requisite variety of insect 

 forms ; the zoologist, to ac- 

 count for the differentiated 

 organs of the insects, as 

 naturally postulates a corre- 

 sponding variety of floral 

 structures. We see, there- 

 fore, that these special re- 

 lationships could only have 

 arisen out of conditions much 

 more simple. Flowers and 

 insects must have become 

 differentiated together. The 

 primitive shallow blossoms 

 and short-Upped visitors would necessarily be in a state of 

 stable equilibrium ; in the absence of any means of isolating 

 those flowers and insects in which corresponding variations 

 happened to take place, whatever variations did occur, would 

 tend to neutralize each other and no evolution would be 

 possible. But in this case, as in artificial selection, the 

 element of volition comes in. The instinct and intelligence 

 of the insects themselves leadmg them to select those 

 flowers which suited them best would, to some extent at 



Fig. 



10. — Angraecum 

 sesQuipedale. 



