173 



Finally, a few words mor© concerning the etiology of 

 SS^- * 5^*^T^^ly. nothing at all concerning the factors ' a ct- 

 ually effective IS indicated by the statement that these galls 

 ore produced after the infection of vegetable tissue by any 

 S^«2^*?JS^ ^''J^^y ^"J^^^r to the plant organs goes hand in 

 hand with infection. Inddod many galls very strikingly re- 

 semble tissues produced after injjrry. This phenomenon becomes 

 Clearly evident in those cases, for instance, in which are in- 

 volved diseased products of the cambium, influenced by para- 

 sites. In very many other galls, the structure of which ex- 

 ceeds the kataplastic character of wound tissue, at least the 

 first stages of development resemble callus tissue. The fact 

 ^^I-*^i^ the production of prosoplasmatic galls, new processes 

 Of differentiation and formation must be added to those known 

 S ^^7 i^^' P^°^®s satisfactorily, however, that still other 

 stimuli besides traumatic ones must often participate in the 

 STl^S^iS'^ ®^ f^^^}®- Besides this, there are many galls in 

 which it can be demonstrated that infection takes place with- 

 out injury to the vegetable tissue, in which therefore all 

 traumatic influences are excluded even from the beginning. 

 The gall stimuli are undoubtedly of a chemical rm*v.t»; some un- 

 known BUDstanoes, excreted by the parasite, incite? tJto colls 

 of tho host plant to ggrowth and coil-division and undnr contain 

 condition^ the products of division to difforont processes of 

 differentiation. We know as yet nothing definite concerning 

 the chemical character of the substances actliig here. Up t» 

 the present, all attempts to cause gra 1 formation by artificial 

 inoculation with substances of known composition have failed. 

 Unfortunately experiments, made with the contents of the "poison 

 sac" of the leaf wasp in the endeavor to call forth "artificial" 

 gajLl formations, have as yet given no positive results*. 



Disregarding the fact that the wound stimuli, preceding th« 

 formation of various galls, enclose within themselves a complex 

 of BEony factors the significance of which is still bxit little 

 known, aa analysis of the factors active in the formation <>f 

 galls was not perfected ^ven with the discovery of traumatic 

 stimuli and of the action of gall poisons. It is to be hoped 

 that future experiments will throw light on the question as to 

 whether the tissues of the plant in their normal condition are 

 always susceptible to chemical stimuli or whether, in the for- 

 mation of galls, they are often made susceptible to stimuli of 

 other kinds only by traumatic interference and its results. It 

 will be necessary to test further the questions as to vrhether 

 the stimuli or contact proceeding from the larvae, which wan- 

 .{193) der about inside the half-grown gall, are not also Important 

 in the further development of the gall, or whether it is pos- 

 sible, and in which cases it is possible, for the larvae to 

 bring wound stimuli into eiffect by grazing upon the innermost 

 .tissues, thereby causing a new production of tissue;- whether 

 the excrement balls, thrown off by tho inhabitants of the 

 galls, can act as a fertilizer, whether new stimuli proceed 

 from these, etc. 



We know but little, at any rate, about the capacity for re- 

 action against gall poisons possessed by definite plents and 

 definite tissue forms, because, in judging of this, we are 

 still dependent upon the investigation of the new formation of 

 tissue produced in nature anft because the gall-insects cause 

 their virus to act on2)y on ^ definite substratum. On this ^a^ 

 count the question, as.to jwhether tissues other than the ones 



^ Beverinck, Ueb, d. Cecidium v. Nematus Capreae auf Saliac 

 amygdalina BAtan. 2tg. , 1888. Bd, XLVI, p. 1. 



