ACTION OF LIGHT ON POST-EMBRYONIC GROWTH. 537 



and it undergoes periodic alterations each day. Evidently, however, in both cases the 

 effects on growth, so far as they occur at all, must be cumulative ; though perhaps in 

 the course of a few hours or of a day unnoticeable, these actions after months and 

 years may produce very considerable effects, which however up to the present 

 are but little known. Since however we already know a large series of such 

 effects, though notice has only quite recently been directed towards them, it may 

 be expected that many more will be discovered; and if it is remembered that 

 while the vegetable world from its origin onwards has been gradually evolving from 

 its lowest forms the highly organised ones-^that during this time all processes of 

 growth have been continually affected by gravitation, and at least periodically and 

 partially acted on by light, it may be presumed that almost all relations of organisation 

 must gradually have been modified to a very large extent by gravitation and light. 

 In other words, the forms and modes of life presented by plants, and which the 

 botanist studies, must have been to a large extent induced by the continued action of 

 gravitation and light. We can now artificially induce or prevent some of these 

 actions, but others are perfectly hereditary and have become constant. Evidently 

 there here lies before us one of the most fruitful provinces for botanical investigation. 



Finally, it remains to mention still a few other cases of the action of external 

 forces on the growth connected with configuration; but this must be done quite 

 shortly, in order not to spin out this lecture too long. I have already referred to 

 cases where by the growth of parasitic Fungi the tissue of the host-plant is not 

 destroyed, but is even enormously promoted in its growth. One of the finest 

 examples is presented by the formation of the so-called " Witchs' brooms " on Firs ; 

 — on the horizontal branches permeated by JEcidmm elatinum are produced abnormally 

 orthotropic shoots, growing like small primary stems. Another category of growths 

 abnormal for the plant itself occur in the numerous gall-formations, so common 

 everywhere; — insects, especially Gall-flies, lay their minute eggs in the tissues of young 

 growing parts of plants, and while the small larva is developing from the egg the 

 vegetable tissue surrounding it becomes hypertrophied, and thus forms the Gall. 

 The common " gall-apple " of the Oak, and the galls of the Rose-bush furnished 

 with their moss-like appendages, and others are known to all. Two points 

 in the formation of galls are especially worthy of note. First, every species of gall 

 possesses a definite anatomical structure and external form, as if the gall were an 

 organism sui generis; and secondly, this complex organisation is induced simply and 

 only by the development of a specifically distinct insect larva, so in fact that entirely 

 different galls are produced on the same plant by different insects. It may here be 

 mentioned by the way that certain aphides produce gaE-like structures, among 

 which the large hollow Chinese galls, and the galls resembling pine-cones produced 

 by Chermes viridis on our Red Firs, are especially noteworthy. It has lately been 

 established by Peyritsch, moreover, that some rnonstrous floral developments, so- 

 called green flowers {Chloranthy), especially of species of Arahis, can be artificially 

 induced by placing certain species of Aphis on the still young inflorescences. 



In these cases it is abnormal external stimuli which induce abnormal growth. It is 

 of very common occurrence, however, not only in the vegetable but also in the animal 

 kingdom, that new growth results in a perfectly normal manner from fertilisation. 

 Here again the important matter is that a material influence acts on the female organ 



