216 CO-OPERATION OF PLANTS. 



may be facilitated, the fig growers take branches from the Capri- 

 ficus variety and hang them on the trees of the true fig, the 

 operation being termed " Caprification." 



Then the case of the Yucca lily and Yucca moth is another 

 interesting instance of symbiosis. The Yucca lily is a handsome 

 plant, growing in California, and bearing pendulous flowers in 

 shape like the tulip. These flowers are visited by the female moth, 

 known as the Yucca moth. This insect seems to make a special 

 object of collecting the pollen from the stamens, rolling this pollen 

 into round balls, and carrying it to another flower. Arriving there 

 the moth, by means of a long tube, bores a hole right through the 

 wall of the ovary, and lays its eggs inside the ovary near the young 

 ovules, then creeping up the style the moth places the pollen 

 which it brought from the other flower on the stigma, and even 

 forces this pollen down the style as far as it can. The ovules are, 

 of course, fertilised by this pollen, which, coming from another 

 flower, causes the ovules to ripen into sound succulent seeds. 

 The eggs meanwhile, becoming hatched, form young moths inside 

 the ovary, and these young insects eat up as food many of the 

 succulent seeds before they escape out to the light. Therefore, 

 there is no doubt that, in return for the depositing of this pollen 

 and consequent fertilising of its ovules, the Yucca plant repays the 

 moth by sacrificing some of its seeds as food to the young moths. 

 Yuccas grown where there are no moths produce no seeds, through 

 no fertilisation taking place. 



I might enumerate many other examples of decided symbiosis 

 between plants and animals, but will be content with the four or 

 five mentioned, as I think these will suffice to prove that between 

 certain animals and plants we have a most interesting system of 

 reciprocity existing. It will, however, be useful to state that the 

 formation of galls on the oak, willow, etc., caused by certain insects 

 laying their eggs, cannot be regarded as furnishing an instance of 

 symbiosis, the advantage, as far as can be seen at present, being 

 all on the side of the animal, so these gall excrescences must 

 therefore be regarded as parasitic structures. 



