253 



old twigs (current year's growth) it was reduced to a few con- 

 tinuous streaks. 



Abundant hyphae were found growing in all directions in the 

 wood elements (see Text Fig. 8), from the cambial region inwards. 

 The hyphae present in the lumen of the vessels were in every 

 respect identical with those observed in other hosts, already 

 referred to, and similar to those grown in artificial cultures of 

 Stereum purpureum. 



There was no opportunity of examining the roots of Laburnum 

 vulgare, and it is unknown if root infection took place. The 

 fact that the upper set of branches of one half of the tree was 

 killed by the fungus, whereas the lowest branches were iipparently 

 normal, indicates that the fungus penetrated through wounds in 

 consequence of pruning, or through the abraded surfaces which, 

 frequently occur in the crown of the tree. 



III. — Modes of Infection. 



According to Percival (4), Giissow (12), Pickering (8), Brooks 

 (9), the fungus Stereum purpureum is a wound parasite. In 

 order to confirm this view several inoculation experiments were 

 carried out in the laboratory. Pieces of mycelium, fructifica- 

 tions, and spores were placed on uninjured leaves of Prunus 

 lusitanica in Petri dishes. Although the conditions for develop- 

 ment of the fungus were most favourable, yet no invasion into the 

 tissue could be observed. If, however, spores, pieces of fructifica- 

 tions and mvcelium are laid on leaves which have been scratched 

 with a needle or from which the epidermis has been removed, the 

 penetration of the hyphae into the host soon begins, followed by 

 partial disorganisation of the cells of the leaf. It is clear that 

 the fungus if it can once gain entry can develop in the leaf, and 

 it is hoped to carry out further experiments on this point. 



Several plants of Prunus lusitanica were inoculated by placing 

 pieces of mycelium of S. purpureum beneath the bark of stem, 

 branches and roots. "Within three weeks, discoloration of the 

 cambial region of the wood appeared on both sides of the point 

 of inoculation. In agreement with Brooks, I observed the rapid 

 growth of the hyphae in an upward direction and a very slow 

 progression down the shoot. Precisely the same process results 

 from the insertion of pieces of fructifications or pieces of mycelium 

 in the roots. In the control experiments no effect at all was 

 obtained. 



Stem and Branch Infection. — From the actual observations in 



the laboratory and in the field it would be premature to state at 

 what particular time of the year infection takes place, since it is 

 not yet known how long the spores retain their vitality. The 

 following observation shows that they are capable of immediate 

 germination. An old silvered branch of " Victoria Plum " was 

 taken to the laboratory in October, and S. purpureum fructifica- 

 tions developed upon it within eight days from the dav of 

 amputation. Basidiospores taken from these fructifications 

 germinated within 24 hours in hanging-drop cultures. Spores 

 are undoubtedly blown about by the wind and possibly alight on 



