1915] 



BURT THELErHORACEAE OF NORTH AMERICA. IV 631 



is stimulated to growth by the presence of the parasitic vegeta- 

 tive hyphae, by absorption of organic products from the host, 

 and, undoubtedly, by excreta from the hyphae. We may see 

 from Woronin's figures that the various organs of a given 

 host produce different galls when infected by the same 

 fungus ; from which we may conclude that the several organs 

 of the host make different growth responses to the same 

 stimulating cause. We have in the host itself, in its several 

 organs, and also in the age of tissues of these organs, as I 

 shall point out later, factors not only able to produce, but 

 actually producing, diversity in gall form even though but a 

 single species of Exohasidium is the parasitic stimulant. Of 

 what value, then, is the form of the gall as a taxonomic char- 

 acter for species of Exohasidium? 



The different organs of the host differ in the resistance 

 which they offer to infection by Exohasidium. Woronin notes 

 in his work cited that out of more than a thousand specimens 

 of Exohasidium Vaccinii, only twelve showed flower galls. 

 Hence the flowers of Vaccinium vitis-idaea are much less sub- 

 ject to infection than the leaves. In only the one case, which 

 he illustrates by fig. 18, did he observe local infection of a 

 flower. In figs. 16 and 17, the infected flowers are borne on 

 infected shoots and may have become infected through these 

 shoots. We may therefore conclude that in a given host a 

 high resistance of certain organs to infection by Exohasidium 

 restricts the galls for that host to fewer organs and to a 

 smaller number of forms than in some other host with a lesser 

 resistance. 



That the age of the organs, or their cells, of a host is an 

 important factor in the determination of gall form is appar- 

 ent if one observes throughout a season the succession of galls 

 produced by a favorable host. In this connection Richards^ 

 has stated, ^'and also on Gaylussacia resinosa in the earliest 

 formed distortions, whole shoots are transformed. Later in 

 the season the Exohasidium forms only slight local distortions 

 on the leaves, and still later one finds forms which do not dis- 

 tort the tissues of the host plant at all, but simply form a 



iBot. Gaz. 21 : 107. 1896. 



