SECTIONAL TRANSACTIONS. -K*. 397 



Mr. G. A. ZahSt and Mr. E. J. Neethling. — Notes on the Exotic Trees in 

 the Cape Peninsula. 



Mr. G. A. Zahn and Mr. C. H. Clayton.— .4 Summary of the Working 

 Plan for Tolcai Plantation. 



Afternoon. 

 Excursion. 



Wednesday, July 24. 



Prof. J. H. Priestley. — The Movement of Water and Solutes in the Tree 

 (Joint Meeting with Section K). 



Dr. E. V. Laing. — The Biology of the Soil and its Relation to Tree Growth. 



The knowledge already acquired regarding the micro-organisms of the forest soil 

 indicates that the subject is of first-rate importance to the silviculturist. Just as 

 the agriculturist has benefited from soil micro-biological researcli, so undoubtedly 

 •will the forester, but whereas the agriculturist is most intimately bound up with 

 bacteriology, the forester is, as far as our present information permits of affirming, 

 dependent more on the action of numerous fungi. The bulk of research has centred 

 round the problem of the intimate union of fungi with tree roots, but that intimate 

 association of bacteria and tree roots also occurs is considered possible. The union of 

 fungi and tree roots illustrates all stages from apparently true beneficial partnership 

 to intensively destructive parasitism. Fungi play an important part in moulding 

 root habit or form. In view of the importance of mycorrhiza the ecology of 

 mycorrhiza-forming fungi is of paramount value. 



Afternoon. 

 Excursion. 



Thursday, July 25. 



Mr. G. A. Zahn and Mr. E. J. Neethling. — The Cluster Pine (Pinus 

 pinaster). 



Mr. W. R. Day. — Environment and Disease : A Discussion on the Parasitism 

 of Armillaria mellea, Vahl. Fr. 



The fundamental causes of disease are to be found in the inhei-ent character of the 

 disea.sed organism and the factors of environment, both physical and biological, with 

 which it has to contend. Parasites, often regarded as the chief cause of disease, are 

 only one factor in this complex of forces bringing about a diseased state, and frequently 

 by no means the most important. When a disease is investigated it is necessary, 

 therefore, to ask what is really its primary cause, and what are the factors which 

 make it possible for this to act. Only when this is understood is it possible truly to 

 appreciate the importance of other factors acting secondarily. If the root-rot of 

 trees associated with attack by A . mellea is examined, what is the role of this fungus 

 seen to be ? The evidence brought forward here all goes to show that it is always a 

 secondary cause of disease infecting and killing trees that have already become 

 debilitated owing to other causes. These are sometimes fairly obvious, such as severe 

 drought, or defoliation by insect larvse ; but in many instances the primary cause of 

 disease is more obscure, and the fungus itself may appear to be the initial cause of 

 disease. However, in no instance where the matter has been investigated has this 

 been found to be true, and the manner in which the susceptibiUt}^ to infection and to 

 death after infection varies goes to confirm the secondary role of the fungus as a cause 

 of disease. With A. mellea, therefore, as with many other parasites, and especially 

 those affecting forest trees, efficient control depends on a proper understanding of 

 t he environment in which the trees groW; and of the conditions necessary for healthy 

 tree growth. 



