182 



Knowledge 

 necessary 

 for a suc- 

 cessful In- 

 vestigation 

 of Plant 

 Diseases. 



Sub-divi- 

 sion of the 

 Subject. 



159. In order that our study of 



tne diseases of our forest plants shall be of practical value, 

 we must by careful observation and study : 



(1) Be able to recognise the first symptoms of a diseased 



condition. This among other things necessitates 

 a knowledge of the life-history of our plants. We 

 should, for instance, be inclined to ascribe such 

 phenomena as the annual dying back of the seed- 

 lings of bamboos and other species, or the shed- 

 ding of shoots of some species of Strobilanthes, 

 to a disease, if we did not know that these are 

 necessary in the ordinary course of the plant's 

 development. 



(2) Be. able from these symptoms to at once form an 



approximate idea as to the nature of, at all events, 

 some of the principal factors causing the disease. 



(3) Be acquainted with, and know the mode of action of, 



all important factors which either beneficially or 

 injuriously affect the normal development of our 

 plants. This will then enable us to discover in 

 any given case of disease any unusual absence of 

 beneficial factors or presence of injurious influ- 

 ences and, while thus indicating the principal factors 

 responsible for the disease, will help us in deciding 

 on the most advisable remedial and preventive 

 measures. 



(4) Be acquainted, as far as possible, with the life-histories 



of plants and animals which are capable of bene- 

 fiting, or of injuring, our forest plants and with 

 the factors which influence the development of 

 the same, so that we may be able to take measures 

 for their destruction, if injurious, or for their 

 advantage, if beneficial. 



160. A detailed account of the 



life 'histories of the various animals which influence the develop- 

 ment of forest plants, their mode of action, the symptoms 

 by which the diseased conditions caused by them may 

 be recognised, together with an account of the measures 

 to be taken for their destruction, or advantage, form 

 the subject of Forest Zoology. It is, however, interesting 

 to note that several cases of symbiosis between plants and 

 animals are known. Insects, for instance, effect the pollina- 

 tion of flowers in return for nectar or pollen received 



the flowers, birds and other animals disseminate seeds in 



