Chap. IX. CONCLUDING EEMAKKS. 281 



he visited a site where Vanilla creeps over almost every 

 tree, and although the plants had been covered with 

 flowers, yet only two seed-capsules were produced. So 

 again with an Epidendrum, 233 flowers had fallen off 

 unimpregnated and only one capsule had been formed ; 

 of the still remaining 136 flowers, only four had their 

 pollinia removed. In New South Wales Mr. Fitzgerald 

 does not believe that more than one flower out of a 

 thousand of Dendrobium speciosum sets a capsule ; and 

 some other species there are very sterile. In New 

 Zealand over 200 flowers of Coryanthes triloba yielded 

 only five capsules ; and at the Cape of Good Hope only 

 the same number were produced by 78 flowers of Disa 

 grandijlora. Nearly the same result has been observed 

 with some of the species of Ophrys in Europe. The 

 sterility in these cases is very difficult to explain. It 

 manifestly depends on the flowers being constructed 

 with such elaborate care for cross-fertilisation, that they 

 cannot yield seeds without the aid of insects. From 

 the evidence which I have given elsewhere * we may 

 conclude that it T^ould be far more profitable to most 

 plants to yield a few cross -fertilised seeds, at the 

 expense of many flowei-s dropping off unimpregnated, 

 rather than produce many self-fertilised seeds. Profuse 

 expenditure is nothing unusual under nature, as we see 

 with the pollen of wind-fertilised plants, and in the 

 multitude of seeds and seedlings produced by most 

 plants in comparison with the few that reach maturity. 

 In other cases the paucity of the flowers that are im- 

 pregnated may be due to the proper insects having 

 become rare under the incessant changes to which the 

 world is subject ; or to other plants which are more 



" ' The Effects of Cross and Self-fertilisation in the Vegetable 

 Kingdom,' 1876. 



