MUTUALISM. 



89 



the case of the so-called " storni-tirs " of the mountains, on 

 which are developed not a single apex, as in the normal fir, 

 but many, each of which grows up like a little independent 

 tree on the branches of the old stem. A similar result follows 

 where a portion of a lateral branch is planted as a " cutting," 

 one bud grows directly upwards, the others form lateral branches. 

 The stimulating effect which the removal of the terminal shoot 

 produces on lateral branches is thus one which extends to a 

 considerable distance. A stimulus of a somewhat similar nature 

 appears to be exerted on buds attacked by certain fungi, so 



Km. lii. — Witches' broom of Silver P'ir, caused by Aecidinm elati 

 (V. Tubcuf pilot.) 



that the shoot produced from such a bud no hjnger retains its 

 iiunual direction of growth, l)ut Ijecomes negatively geotropic 

 like an independent plant. This marked negative geotropism 

 is characteristic of all witches' brooms (Fig. 16), and shows 

 clearly that they are no longer controlled by the same laws of 

 growth as the normal lateral branches. They have in addition 

 other peculiarities not exhibited by normal plants. Thus tlie 

 witches' broom of the silver fir caused by mycelium of 

 Accidium clatinum is not evergreen, but bears needles which 

 fall each autumn. Moreover, no witches' broom bears fiowers 

 or fruit ; for example, that on the cherry (Fig. 5) produces 

 exclusively leaf-buds which unfold simultaneously with the 



