56 DISEASES OF CONIFEKS. 



stems, leaves, etc., of the Conifer, and so bring about the death 

 of the whole or of parts of it ; and, on the other hand, there 

 are dangerous physical conditions of the soil, climate, atmosphere 

 and so forth, which render the life of the Conifer more or less 

 precarious or even impossible. 



As a matter of fact, however, these two classes of dangers are 

 frequently found acting together, and so a given case of disease may be 

 complicated owing to the co-operation of many factors. In other 

 cases it is found that the symptoms known to be characteristic 

 of a particular disease are so closely simulated in diseases due 

 to quite other causes than those which produce the primary malady, 

 that confusion results, arid barren lines of action are started by 

 the practical man who fails to discriminate between the various 

 cases. 



Instances of this kind are so instructive that we may take as an 

 example the well-known disease of Pines characterised by premature 

 shedding of the leaves, as yellow and brown needles, which collect 

 in dense heaps beneath the trees. In some cases it is certain that 

 the leaves of young Pines are cast suddenly, and in dangerous 

 quantities, after a sharp frost, or at least after a night so cold that 

 the still soft foliage is chilled below a point which we might call 

 the death-point for these organs. In other cases, however, similar leaf- 

 casting occurs under conditions which are very different in their 

 action. Young Pines suddenly lose their " needles " in warm sunny 

 weather when the ground is frozen hard ; or these organs fall in 

 showers after a period of drought in a hot summer. 



Now although the symptoms which preface and accompany the above 

 cases of premature leaf-casting are in the main similar the green 

 leaves turn yellow, and then brown, and rapidly fall, shrivelling in 

 heaps, to the ground below the disease is a different one, and is 

 caused by different agents in each instance, and it is even possible 

 to obtain fairly obvious evidence of this. In those cases where the 

 fall is due to the direct action of frost or of cutting cold winds 

 i.e., where the leaves are killed by the sudden abstraction of heat 

 from their tissues keen observers have found that those basal parts 

 of the " needles " which are enclosed in and protected by the 

 sheathing-scales of the short branches (" tufts ") may remain fresh for 

 some time after the exposed parts have turned brown and shrivelled 

 up. In the second class of cases, however, no such partial shrivelling 

 of the leaves is seen ; the tissues dry up all along the " needles " 

 from tip to base completely, and this is because they have been killed 

 by drought either because the roots in the frozen soil cannot supply 

 water to replace what is being transpired in the bright sunshine, 



