956 



THE NATURE BOOK 



which the cross- 

 fertilisation of such 

 a complicated 

 flower is carried 

 out present many 

 points of interest. 

 The story of the 

 cross - fertilisation 

 of the Early Purple 

 Orchis {Orchis 

 mascula) is one of 

 the most fascinat- 

 ing in the world 

 of plants. An ex- 

 amination of the 

 flower will show 

 that the petal 

 forming the lip is 

 carried backwards 

 into a spur which, 

 although it contains 

 no nectar, is valued 

 as a food substance 

 in itself by bees. 

 Now the head of 



the insect in search of the succulent tissue 

 comes into contact with two sticky discs 

 attached to the stalks of the pollen 

 masses ; so that when the bee emerges 

 the pollinia are carried away. 



At first the pollen masses stand erect, 

 but in about half a minute, owing to a 

 beautiful arrangement whereby the base 

 of the stalk contracts, they incline forward. 

 Thus by the time the bee has arrived at 

 the next flower the pollen masses are in 

 such a position that they cannot fail to 

 strike the stigma. The manner in which 

 the pollen masses are attached to the 

 heatl of the bee may be well seen if the 

 point of a pencil is inserted into the 

 cavity of an orchid flower, and then gently 

 drawn away. 



It is not always that the plant, after 

 attracting the insect to pay a visit, is 

 willing to let the creature take its departure 

 at once. In the case of Aristolochia 

 clcmatitis, gnats and other insects are 

 sometimes held prisoner for sixty hours 

 until the work which is required of them 

 has been performed. The essential organs 

 are contained in a kind of bulbous swelling 

 at the base of the flower, to which entrance 

 is only possible through a narrow tube. 

 This passage way is lined with stiff bristles 

 pointing downwards, through which the 



THE POLLEN MASSES MAY BE REMOVED 

 FROM THE FLOWER OF THE PURPLE 

 ORCHIS ON THE POINT OF A PENCIL. 



incoming insects 

 are easily able to 

 force their way. 



When the flies 

 arrive the stigmas 

 are in readiness to 

 receive the pollen 

 which the visitors 

 will bring with 

 them ; meanwhile 

 the insects are re- 

 galing themselves 

 on the tissue which 

 forms the walls of 

 the apartment in 

 which they are 

 really prisoners. 

 When they think 

 they would like to 

 depart and make 

 attempts to fly up 

 the tube, it be- 

 comes evident that 

 no escape is possible 

 through the hairs, 

 which completely bar the way. It is not 

 until the end of three days that the anthers 

 have come to perfection, and the flies in 

 their frantic efforts to escape become 

 well dusted with pollen, that the barrier 

 dries up and the captives are allowed to 

 go on their way. 



Among our hedgerow plants the 

 common Cuckoo Pint is an interesting 

 study of the way in which insect visitors 

 are held captive. W^ithout pulling aside 

 the greenish spathe of the arum it is 

 impossible to see the true flowers, which 

 are clustered round the base of the up- 

 right column called the spadix. At the 

 bottom of all are to be found the female 

 flowers, and just above them is the ring 

 of male blossoms. Higher still we find 

 a mass of hairs pointing tlownwards. The 

 ovary-bearing blossoms mature first, and 

 at that time give out rather an un- 

 pleasant odour which is peculiarly attrac- 

 tive to flies. The insects arrive in 

 numbers, many of them well dusted with 

 pollen from other arums which have 

 been visited, and find it an easy matter 

 to go down through the palisade of hairs. 

 After visiting the female flowers, each of 

 which produces a drop of honey, the little 

 visitors do not find it such a simple 

 business to escape upwards through the 



