140 .1. Kki.txvaax — DocTKiJS \'. Lkktwkx 



g'wA] uioniHls . l)ut tliat ('xiK'iiiiicnts (»iily ciUi iiijikc out whether 

 different \> ;i j» ii v e r s uct .ualls from one speeies of A)(1a.r or whether 

 each sort of 1* a ]». has an Auiti.r of its own , wliich Auhixes differ 

 in biolooieal cliaraeters from eacli other. 



Tlie first wonhl he t-nie , i. e. tliat one sjK'cies of Aulax could 

 make galls on \arions Ta]) a vers, if we could show that tor in- 

 stance the AnUt.v of Pai», d u b i u m could ]>rodnce j>alls on P a j). 

 r h o e a s, a r «i' e m o n e an<l \ ice versa. 



For two reasons this seems \-ery imi>rol)al»le to ;is and it is of 

 importance that our observations in the o])en country led to the 

 same cimsiderations as our exi)eriments in the <>arden. In the tiist 

 place we found in Heilo near Alkmaar some fields with P a p. d u- 

 bium, whicJi carried iimid)(n\s of yalls (»f Aiiltix pop. There were 

 fields on which oakcoj)ses had been cut, afterwards the <iround had 

 been planted with potatoes and then been left alone. The followinfj- 

 year there were thousands of Pa]), d u 1) i u m and also an abun- 

 dance of ji'alls. Some I^ a [>. r h o e a s stood 'oetween them, but on 

 these no <>al]s were discovered. 



The year after this s[>ot was almost com])letely shadowed over 

 l)y the oak - plants and some other ]>lants quickly coming- up , as 

 L y c h n is di u r n a , A n t h r i s c u s sylvestris and many 

 kinds of thistles and Uiost of the P a p a vers were not develo])ed. 

 Thousands of üalls, had fallen on the oround and so in due time 

 millions t)ï Anlax must ha\'e swarmed there. Though on visiting the 

 s])ot we could (d)serve the females laying eggs in the remaining 

 P a p. d u b i u m and we found the galls afterwards, there was no 

 question of any galls on P a )>. r h o e a s, which even stood there in 

 greater number than P a ]>. d u b i u m. This year the P a p a vers 

 are numerous on another field , treated in tlie same way at half a 

 mile' s distance from the (dd one and there also the A«/r/.r-galls are 

 (îommon, but on P. R h o e a s no gall is to be found. Of course, if 

 P. ß h o e a s were also infected, it would not be certain , that the 

 galls came from the same Aulax. Only ex)>eriments could make this 

 certain but as the results here are negative, the proof is the stronger. 



In Be^'erwyk Ave w<ne able to observe something like this; Pap. 

 d u h i u m and a i' g e m o u e grew side by side in abundance; but 

 though on the fiist ]»lant galls were nuud)rous , they were missing 

 on the second. So we take it for Ncry improbable, that one kind ot 



