204 LECTURE XII. 



Gall-formations are also to be added here. By means of the stimulus exerted by 

 insects during their development in the interior of the plant the growing parts of the 

 plant may develop in monstrous forms, or bodies of very peculiar sharply defined 

 form grow forth from them. In this connection the fact is of special interest 

 that the quality of these galls on the same plant depends chiefly upon the specific 

 peculiarity of the animal which produces the gall by its irritation. On our Oaks 

 alone, more than a dozen different forms of galls are produced by different insects. 

 Nevertheless, although very common, these phenomena are more isolated and 

 incidental. The most remarkable and beautiful dependence of the highly-organised 

 flowering plants upon insects makes itself evident in the mechanisms of flowers, 

 discovered by Conrad Sprengel in 1794. Sprengel showed even then that all 

 beautifully formed and coloured and odorous flowers, in all their relations of con- 

 figuration, are adapted for being visited by insects of a certain form and size for 

 the sake of their nectar; at the same time these animals transport the fertilising 

 pollen from the anthers on to the stigmas of other flowers of the same kind. Since 

 the development of seeds is only complete in these cases, the reproduction of these 

 plants depends upon the visits of insects ; as, again, on the other hand, the entire 

 existence of the insects referred to is conditioned by the flowers of these plants. 



Even this small selection of examples will suffice to show how the whole life of a 

 plant — not only with respect to its origin, but also to its maintenance in the widest 

 sense of the word — depends upon external influences of the most various kind. It would 

 be incorrect to suppose, however, that the vegetable world as such, or any individual 

 plant-form, can be called into life at any time whatever by these external causes. All 

 that we have here considered are simply and solely reactions of vegetable substance 

 already existing, towards external influences on the same. The mode in which these 

 reactions are manifested depends, however, upon the nature of the given plant; 

 and a further problem of physiology — and, indeed, the most difiicultone of all — lies 

 in the investigation of this innermost nature of the plant, by means of which it is 

 rendered capable of these reactions. 



