AND CROSS-BREEDING. 145 



remove the pollen of flowers, which is less known, and the 

 object of which in the vegetable economy is not so evident 

 viz., by acually eating it. This has chiefly been observed in 

 the case of flies or Diptera belonging to the class Syrphidae, 

 the movements of which in summer and autumn, in hovering 

 over flowers and then suddenly darting upon them, are so 

 remarkable. Many entomologists doubt whether it is possible 

 for flies, which have no mandibles, and whose only food- 

 obtaining organ is a proboscis adapted for suction, to masticate 

 so comparatively hard a substance as pollen-grains. This 

 need not, however, present a difficulty to any one who has 

 smarted under the irritating attacks of flies and midges 

 during rainy weather in mountainous countries. We have 

 ourselves dissected the bodies of flies belonging to this family, 

 and found their stomachs in many cases perfectly loaded with 

 pollen-grains. Prof. Miiller takes this view very decidedly, 

 and gives some admirable drawings to show the manner in 

 which the extreme tip of the proboscis is furnished with a. 

 number of cross-bars, by means of which, as he has himself 

 observed, these insects are able even to tear asunder the fine 

 threads by which the grains of pollen are frequently attached 

 to one another, as in the Evening Primrose. It is often a 

 matter of surprise to the cultivators of flowers that many 

 species which flower luxuriantly in our gardens never produce 

 fruit or seed, though all the separate organs of the flower 

 appear to be perfectly developed. This is the case, for % 

 instance, with the large white Convolvulus grown frequently 

 against the walls of houses, and with the yellow Jessamine 

 which flowers in the winter, and to a less extent with the 

 Calycanthus or Allspice tree. The reason of this is, no doubt, 

 generally the absence of those insects which serve as their 

 fertilisers in their native country, our native species either not 

 being attracted by their foreign nectar, or not possessing the 

 mechanical appliances necessary to obtain it, and hence not 

 visiting the flowers. We mentioned at the outset that, though the 

 large majority of flowers are cross-fertilised, yet there are excep- 

 tions to the rule. Darwin has described the peculiar contrivance 

 by which self-fertilisation is effected in the singular Bee-orchis 

 (Ophrys apt/era] of our chalk-hills, alone among our native 

 Orchids. There are not a few flowers which never or scarcely 

 ever completely open their petals so as to allow either the 

 entrance of an insect or the escape of the pollen. An instance 

 of this is furnished by the pretty little bog-plant the Sundew 

 (Drosera rotundifolia). 



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