THE DISEASES OF CONIFERS. 



125 



or gain in force (like all scientific generalisations) according as 

 few or many well-establislied observations are borne in mind. 



Speaking broadly, there are two great classes of diseases 

 which imperil the life of Conifers. There are, on the one hand, 

 diseases due to the more or less directly injurious action of other 

 living organisms — animals and plants — which injure or destroy 

 the roots, stems, leaves, &c., 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 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, and barren lines 

 of action are started by the practical man who fails to dis- 

 criminate 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. 



It 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. 



