CHANGES OF SHAPE DUE TO SYMBIOTIC INTERACTION 171 



One of the best instances of the morphological influence of conjunctive 

 symbiosis is afforded by lichens 1 , for here two organisms unite to produce 

 another of definite shape. Similarly the parasitic Aecidium euphorbiae 

 produces peculiarly-shaped stems on the Euphorbia Cyparissias attacked 

 by it. This shape is maintained in all new growths reached by the fungal 

 hyphae, so that occasionally an unattacked branch will retain its normal 

 appearance. The witch's broom of the fir due to Aecidium elatinum, 

 that of Pteris quadriaurita due to Taphrina Laurencia, and the peculiar 

 growths on the plum due to Taphrina Pruni, afford additional instances 

 of the results produced by penetrating fungi 2 . Probably all plants 

 respond more or less to the penetration of fungi, although only an 

 internal change or a slight external one may result 3 . Localized stimulation 

 results in the formation of various galls, which when produced by animals 

 may be termed zoocecidia, and when by plants phytocecidia, or myco- 

 cecidia if due to fungi. These afford good examples of how a special 

 stimulus may result in the production of a definite abnormal shape, or even 

 of metabolic products which are not normally formed. Owing to the 

 action of fungi and of insects upon the primordia of flower-buds, green, 

 doubled, or abnormal flowers may result 4 . Abnormalities of this kind 

 may, however, also occur owing to divergence from the normal course of 

 development, which may be indirectly due to peculiarities in the external 

 conditions. 



In some cases of conjunctive symbiosis, the one symbiont lives 

 actually within the protoplast of the other. Thus the galls on Vaucheria 

 are produced by a small rotifer which lives inside the cell-filament of 

 this plant 5 . The same applies to the bacteria which live, grow, and divide 

 inside the cells of the root-nodules of Leguminosae. Further, the hyphae 

 of endophytic Mycorhizae penetrate the living cells of the root, and the 

 small algae which live in the cells of Hydra viridis^ Convohita, and in 

 certain Radiolaria perform the same function as the chloroplasts in a 

 chlorophyllous plant cell. When such an association is hereditary in 

 character, the peculiarities of shape due to it are permanently retained 

 in the offspring. 



1 Peirce (Proceedings of the Californ. Acad. of Sciences, 1899, I, p. 236 ; American Naturalist, 

 1 900, Vol. xxxiv, p. 244) regards lichens as an instance of antagonistic symbiosis, but this does not 

 affect our present considerations. The same applies to Mycorhizae, on which cf. Stahl, Jahrb. f. wiss. 

 Bot., 1900, Bd. xxxiv, p. 618, and also Macdougal, Annals of Botany, 1899, Vol. xm, p. 41. 



2 Frank, Krankheiten d. Pflanzen, 2. Aufl., 1894, Bd. I. 



3 Wakker, Jahrb. f. wiss. Bot., 1898, Bd. x, p. 87 ; Molliard, Rev. ge"n. d. Bot., 1898, T. x, p. 87 ; 

 'Strasburger, Biol. Centralbl., 1900, Bd. XX, p. 657. 



4 Cf. Goebel, Organography, 1900, I, p. 194; Vochting, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, 

 p. 470; Peyritsch, Sitzungsb. d. Wien. Acad., 1898, Bd. xcvil, I, p. 597. 



5 Frank, Krankheiten d. Pflanzen, 2. Aufl., 1895, Bd. in, p. 12 ; Rothert, Jahrb. f. wiss. Bot., 

 1896, Bd. xxix, p. 525. 



