— 405 — 



this sudden change was a marie stratum. The diameter of this 

 basic oasis was not more than about 3 km, and coinciding with 

 the fact that the soil had become acid again the large blue flowers 

 reappeared (A x ). 



Another striking change was observed in Himmerland, as 

 the following diagram wall show: 



[basic] [neutral] [faintly acid] 

 C — > E — — > D > H — > J -> 0 



[parv, albid alone] [all combinations] 



The distance between D (Bælum) and H (near Vorgaard) 

 was not more than about 1 km, but the populations C — E — D were 

 from the high lime districts and the populations H — J — O from 

 the sandy valley of one of the rivers from the glacial period. I 

 was not aware of this boundary line before Viola itself reminded 

 me of it when I saw the large, blue flowers again. 



Ï do not believe that the cause of this distribution is, that 

 tricolor is incapable of growing in basic soil and arvensis incap- 

 able of growing in acicl soil. But plants that react upon the 

 degree of acidity have an optimum at which they become most 

 vigorous (Carsten Olsen 1921), at other concentrations of p H . 

 they become more weak. Apparently arvensis has its optimum 

 in the basic or neutral soil and tricolor in the acid soil. The strong 

 competition in nature then causes that they usually only can 

 pull through when the reaction of the soil approaches the opti- 

 mum for the species in question. 



There are 3 exceptions in table I (pag. 378). The population 

 F with grand, viol on basic soil and T and U with parv, albid 

 on acid soil. When the number of exceptions is so relatively large, 

 it has its natural explanation in the fact that I have selected 

 those populations which I suspected would deviate from the 

 rule. The population F is from Holmsland (not Holmsland 

 Klit) between Ringkøbing Fjord and Stadil Fjord. It is a fertile 

 basic oasis with loamy soil in the West- Jutland acid and sandy 

 heath land. There is marie on Holmsland. Seed and grass was 

 \ery vigorous, likewise Viola. I did not succeed in finding one 

 arvensis here. 



The population T is from a stony rye field near the harbour 



