PREFACE. Ill 



economic importance which they contain. If a future edition of 

 this Manual is called for it will probably be then advisable to 

 add a section dealing with this part of the subject. 



5. It is to be hoped that in a few 



years' time our knowledge of injurious fungi will be suffi- 

 cient for the compilation of a fully illustrated hand-book des- 

 cribing the most important forest fungi and paying particular 

 attention to characters which can be recognised in the forests. 

 For the present, however, the correct identification of any 

 particular fungus depends on skilled microscopic work and 

 must be left to the expert. The paragraphs of this book which 

 deal with fungi may therefore be considered unnecessarily 

 detailed for students who have little opportunity for working 

 with the microscope. The facts regarding fungi given in Parts 

 /Fand V and the plates illustrating the same, however, merely 

 aim at giving an idea of the general appearance of some typical 

 fungi (the majority of which also the student will have oppor- 

 tunities of seeing and becoming familiar with on his tours), 

 of their life history and injurious action on the plants attacked 

 and also of the microscopic characters on which their classi- 

 fication depends, with the object of helping the student to 

 recognise in the forest the presence of an unknown injurious 

 fungus, to select specimens of the same necessary for its identi- 

 fication by the expert and to form some idea as to the best 

 remedial and protective measures to be taken pending the 

 identification of the pest. 



6. The question of Plant Dis- 

 eases has perhaps been treated on rather broader lines than 

 is usual in an elementary text-book. The principle that a 

 plant's welfare depends on the balance struck between the effects 

 of a numbei of injurious and favourable factors and that a 

 disease is rarely due to a single factor frequently appears to be 

 insufficiently appreciated, while, until we know more about the 

 normal physiology and life history of our important forest species 

 and about the relations which exist between them and other 

 organisms, plants as well as animals, very little real progress 



