[Vol. 2 

 632 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



scurf on the lower side of the leaves. The same succession is 

 found in the forms on Andromeda down to the last men- 

 tioned." Richards determined by culture experiments that 

 the remarkable bag galls of Andromeda ligustrina are merely 

 early (June in Massachusetts) productions under the same 

 specific fungous stimulus which later in the season induces 

 leaf concavities on this host. The account of his experiments^ 

 may be summarized as follows : During July, Exobasidium 

 spores were removed with suitable precautions from fresh 

 mature bag galls of Andromeda ligustrina and were im- 

 mediately transferred to buds and young leaves of experi- 

 mental plants of the same species, w^hich were isolated in a 

 moist chamber. In about ten days faint discolorations of the 

 leaves were noticed, at first yellowish and then pink. About 

 five days later, the spots which had considerably enlarged, 

 began to show unmistakable signs of thickening, forming the 

 peculiar concavities in the leaves seen in other Exobasidia. 

 In external form, and also in the matter of basidia and spores, 

 this distortion resembled precisely the leaf form on Andro- 

 meda ligustrina, and indicates that the Exobasidium which 

 produces the bag galls of the young buds is identical with the 

 fungus which produces the leaf form found later in the 

 season. 



The foregoing presentation of the Exobasidium gall as a 

 growth response of the host under stimulation by the fungus 

 shows that very different forms of galls and differences in 

 regard to abundance of each form on a host may result — 



(a) From the different organs making the response. 



(b) From differences in resistance of the several organs, 

 which, in many cases, may undoubtedly be so great as to give 

 complete immunity for certain organs. 



(c) From the age of the organ attacked. 



Since the host produces a great variety of gall forms as 

 growth responses to attack by a single species of Exobasidium^ 

 how are we to decide whether a given gall form is ever suffi- 

 ciently distinct to entitle its causative organism to separate 

 specific rank? Gall forms are host products to so large an 



^ loc. cit., p. 105. 



