2o8 TRANSACTIONS OF ROVAL SCOTTISH ARBORICULTURAL SOCIETY. 



Numerous observations have been made by Haak and Dr 

 Dugler as to how the mycelial hairs enter the leaves and how 

 they spread. These observations are most delicate and 

 difficult, and certain phases are still to some extent conjectural, 

 but it is evident that the leaves in full growth can resist the 

 full development of the mycelia. Though the disease cannot 

 be shaken off, it is retarded till the growing period of the 

 leaves is over. During the late autumn and winter months 

 the mycelium penetrates in all directions, and with the 

 appearance of spring the leaves have lost all power ot 

 resistance, turn quite red, fade away and are shed. 



The infection and course of the disease in young plants is 

 comparatively simple, and the question naturally arises why 

 older trees of the same species are not attacked in a similar 

 manner. The reason is not that they are beyond the region of 

 infection, for in places where no leaves in new cultures had 

 escaped, older plants, the branches of which swept the soil of 

 the very same area, showed not a single red leaf. Moreover, 

 it has been found impossible to cause the infection of even the 

 newest shoots of older pines by tying leaves with ripe apothecia 

 to them. 



The young pine gradually outgrows danger from the disease, 

 and becomes immune somewhere between the seventh and 

 tenth year. Why this should be is still dark. The reason 

 is certainly not to be found in differences in the anatomical 

 structure of the leaves, but is evidently a biological effect, 

 due to a power of resistance against the entry or growth 

 of the mycelium in tlie leaves of older trees, for as soon as 

 this resistance is lowered the fungus attacks them as well as 

 any young growth. 



This fact is borne out by numerous observations and experi- 

 ments. The sap supply need only be interrupted, causing a 

 gradual cessation of growth, and the leaves of the branches thus 

 treated become as liable to infection as those of any young 

 growth. Even the leaves of branches of felled trees, owing to 

 the fact that they dried up slowly in the shade of dense beech 

 undergrowth, were found covered with ripe apothecia. Numerous 

 apothecia were found on the leaves of trees broken by snow. 

 It may also be accepted that, though the growing leaves of 

 older trees resist any attack of the scab spores, these may 

 establish themselves on leaves in which the sap circulation has 



