CHANGES OF SHAPE DUE TO SYMBIOTIC INTERACTION 173 



probably also formed by the egg itself, for the gall does not attain its full normal 

 size when the egg is removed. 



The enlargement, movements, and gnawing of the larva may also act as stimuli, 

 and these may be of primary importance in certain cases. It is, however, certain 

 that a specific stimulatory substance is not required for each form of growth, for, 

 as we shall see later, dissimilar ends may be attained by the same agency according 

 to its mode of application, the character of the organ affected, and other circum- 

 stances. In the case of galls it has not yet been determined whether different 

 stimulatory substances are produced by the various gall-forming insects. 



Since the gall is formed by the plant, it is only natural that those produced 

 by the same insect on different organs or on different plants should exhibit certain 

 dissimilarities \ Similarly, since different insects exercise unlike stimuli, the galls 

 produced by different insects on the same plant are naturally widely dissimilar. 

 The galls produced by the two life-cycles of such insects as Cynips calicis may 

 also differ not only as to their position but also as to their shape and character. 



The similar shape of the same kind of gall is an evidence that corresponding 

 conditions existed during development. In the first instance this is due to the 

 deposition of the egg in the same manner, and on similar organs, by a particular 

 insect. Usually this is in young parts, but the galls of Cynips Reaumuriana are 

 formed on nearly adult oak leaves, the tissues concerned being awakened to 

 renewed activity. A similar awakening stimulus is necessary to induce the 

 germination of the seeds of Orobanche and of the spores of certain parasitic fungi 2 . 

 Moreover the penetration of pollen-tubes may, as in the case of fungal hyphae, 

 act as a stimulus to growth without any fertilizing influence being exercised. This 

 causes the carpels of many plants to enlarge, and Miiller-Thurgau 3 has in fact 

 found that the penetration of the pollen-tube is required to bring the seedless fruits 

 of certain pears, apples, and grapes to perfection. The same stimulus is required 

 for the formation of the ovules in orchids, and the penetration of pollen-tubes 

 incapable of fertilization suffices for this. Treub indeed found that the same 

 result was produced by the boring of certain larvae into the carpels *. 



SECTION 49. Grafting and Graft-hybrids. 



Many allied plants can be grafted together provided they are capable 

 of maintaining the relationships required for their mutual growth 5 . These 

 relationships must on the whole correspond to those existing between 

 the root and shoot of the intact plant. For example, an insufficient 



1 In regard to gall-mites cf. Frank, 1. c., p. 55. On the overlapping of two different galls 

 Beyerinck, 1. c., 1882, p. 173 ; Kiistenmacher, 1. c., p. 93. 



2 According to Bernard (Rev. gen. d. Bot., 1900, T. xn, p. 108) endophytic fungi excite 

 rination in various orchids and lycopods. 



3 Miiller-Thurgau, Landw. Jahrb., 1898, pp. 25, 61. 



* Treub, Ann. d. Jard. Bot. d. Buitenzorg, 1882, T. Ill, p. 122. 



8 See Vochting, Ueber Transplantation am Pflanzenkorper, 1892 ; Sitzungsb. d. Berliner 

 Akademie, 1894, p. 705. Also Daniel, Rev. ge"n. d. Bot., 1894, T. vi, p. 5 ; 1897, T. ix, p. 213 ; 

 1900, T. xn, p. 355 ; Compt. rend., 1897, T. cxxiv, p. 229, and T. cxxv, p. 661 ; Ann. d. sci. nat., 

 8 e se>., T. VIII, p. i. Cf. also Frank, Krankheiten d. Pflanzen, 2. Aufl., 1894, I, p. 117. 



