320 METAMORPHOSIS 



We have yet to glance at the effect of other organisms on plant shape. The 

 plant can no more escape from the influence of such associations than it can 

 from heat or gravity, since, wherever organisms occur, there we find a competi- 

 tion for space, for light, and for food. The result of this contest is that many 

 of the organisms are killed off because the survivors have prevented them from 

 obtaining the necessary supplies of food material or light. The action of the 

 organism in these cases is only to induce chemical or physical alterations in the 

 surroundings, and these, in their turn, bring about such effects as we have already 

 made ourselves acquainted with. On the other hand, if an organism injures the 

 plant, if it be partly eaten by an animal, for instance, changes occur in the plant 

 body not merely in consequence of the injury as such, but as the result of the re- 

 action of the plant to it. The direct effect of injury needs no elucidation — the in- 

 direct effects will be treated of in the next lecture. The relations existing between 

 the plant and animal worlds are well known, such as those associated with the 

 transference of pollen by insects and the phenomena seen in myrmecophilous 

 plants. There can be no doubt that plants are adapted to the visits of insects, 

 and have become altered in shape accordingly ; but this is a phylogenetic phe- 

 nomenon on which as yet no experimental research has been carried out, so 

 that we may omit any consideration of it. 



In addition to the physico-chemical influences of the organism on the 

 environment above referred to and the related effects of the presence of other 

 organisms, there remains for consideration the direct influence of one organism 

 on another, as when two organisms live together symbiotically or antibiotically. 

 Here also the alterations in structure and appearance, often very remarkable, 

 may be referred ultimately to chemical and mechanical factors, but it is im- 

 possible to treat of these problems comprehensively, because we do not know 

 how these factors act individually, and also because in nature they emanate 

 from the organisms themselves. We may, however, quote a few examples of 

 the morphogenetic effects which arise from symbiosis or parasitism. 



Variations from the normal plant shape are induced by the action of certain 

 parasites, and known as galls. It is impossible to cite more than a fraction of the 

 extensive literature on galls; the following references must therefore suffice : — 

 HoFMEiSTER, 1868 ; GoEBEL, 1898 ; EcKSTEiN, 1891; KusTER, 1903. Fungi are 

 the most prominent causes of galls in plants, and in addition Bacteria (p. 238), 

 Myxomycetes, and Algae ; parasites of higher rank are usually not counted as 

 gall-producers, although they also may induce 'variations from the normal plant 

 shape '. Among animals we take account first of all of gall-wasps and gnats, but 

 there are other insects, as well as worms, which also produce galls. Let us con- 

 sider first of all fungus-galls. The effect of the fungus on its substratum may 

 be to destroy the immediate region affected or, in the long run, the whole plant. 

 That this is due to the activity of a poison given off by the fungus is obvious, and 

 there are cases known where the poison has been isolated, as, for example, oxalic 

 acid (De Bary, 1884 ; Reinhardt, 1892). Such a radical procedure on the part 

 of the fungus is, however, not to the purpose, since rapid growth of the fungus 

 exposes it to injury by destroying its host-plant and finally itself. The behaviour 

 of other Fungi, however, is much more to the point since they do not injure 

 their host-plants at all or may even stimulate them to more vigorous growth. 

 Many Uredineae, and also Erysiphe guttata, stimulate the formation of chloro- 

 phyll in the host ; the Synchytrideae cause the epidermal and neighbouring 

 cells to grow vigorously in size, while others cause swelling of entire internodes, 

 leaves, fruits, &c. Where such hypertrophies are induced we speak of genuine 

 gall-formation. Anatomical investigation shows a great enlargement of the 

 parenchyma cells in these hypertrophies and often also an increase in number 

 of these, and the development in all of them of abundant protoplasm and starch. 

 These alterations are all to the benefit of the fungus as supplying it with extra 



