172 THE CAUSE OF SPECIFIC SHAPE 



As in a typical plant, the life and growth of a symbiotic organism 

 result from the interaction of the physiologically and morphologically 

 dissimilar parts with one another and with the external medium. When 

 the symbionts can be cultivated apart, the effects due to their interaction 

 are readily ascertained, and in addition to the general relationships required 

 for maintenance, locally restricted stimulatory actions may be exhibited. 

 The formation of galls is the result of a purely local stimulus, although 

 in the ultimate resort the entire organism is unavoidably affected to a 

 greater or less degree. The development of the egg and larva of the 

 gall-insect is, however, dependent upon the response made by the plant and 

 upon its productive activity. In every case, in fact, reciprocal influences are 

 exercised both here and in cases of disjunctive symbiosis, as when the 

 nutrition and development of a parasitic mycelium is influenced by 

 the character of the tissues of its host. Changes of shape do not, however, 

 always result, and a morphological effect can only be exercised when cells 

 capable of growth or of being awakened to growth are present. 



The above instances of symbiosis show that self-regulatory interaction 

 between symbionts involving the maintenance of definite shape is possible 

 without the existence of any interprotoplasmic connexion. Special metabolic 

 products may influence the character and mode of growth to a marked 

 degree, and in fact the shape of certain galls is due to the action of 

 substances excreted by the ovipositor of the gall-insect. 



Galls are in some cases produced merely by a local retardation or acceleration 

 of growth, in others by a formation of hairs ; they may be closed or open, and 

 frequently are extremely complex in character. A few details may be given here, 

 more especially in regard to the production of galls by the Cynips family 1 . In this 

 case the gall is produced by the parent insect, which deposits the egg in a particular 

 way upon the surface of the plant or in its tissues. The galls of Nematus capreae, 

 which are especially common upon Salix amygdalina, are due to the excretion 

 of a drop of slimy liquid into the wound formed by the ovipositor, and this alone 

 is capable of producing a small gall in the absence of the egg. In other cases, 

 however, if the egg is removed or killed, no trace of gall-formation occurs, for 

 the stimulus is here due to the egg, and in part to the developing larva. 

 Since neither the mere wound nor the presence of a dead egg is able to cause 

 the formation of a gall, the stimulus must be due to some metabolic product, 

 temporarily or continuously produced by the egg or by the larva. In the case 

 of Nematus capreae this substance is deposited with the egg, and some is 



1 Cf. Frank, Krankheiten d. Pflanzen, 2. Aufl., 1894, Bd. m; Eckstein, Pflanzengallen u. 

 Gallenthiere, 1891 ; also Beyerinck, Beobacht. uber Cynipidengallen, 1882 ; Bot. Ztg., 1885, p. 304; 

 1888, p. i ; Ueber Gallenbildung u. Generationswechsel bei Cynips calicis, 1896 (repr. from Verh. 

 d. Koninklijke Akadem. d. Wetenschappen te Amsterdam) ; Kiistenmacher, Jahrb. f. wiss. Bot., 1894, 

 Bd. xxvi, p. 85 ; Appel, Ueber Phyto- und Zoomorphosen, Diss., Konigsberg, 1899; Kiister, Flora, 

 1900, p. 117; Biol. Centralbl., 1900, Bd. xx, p. 531. 





