181 



directly injurious (parasites), and in cases of close symbiosis it 

 is often difficult to decide whether or not one organism is slight- 

 ly injured by the other, that is, whether they are to be regarded 

 as reciprocal symbionts, or as parasite and host. 



157. The symptoms which indi- Symptoms 



catea diseased condition are of course very various 01 

 according to the different factors at work and may 

 consist of unusual pallor of the leaves, discoloured spots 



or blotches on the leaves, premature death of leaves, or 

 branches, and so on. At the same time such symptoms in 

 themselves by no means necessarily indicate the factors which 

 are responsible for the disease. Premature death of twigs 

 and branches may, for instance, be due to the effects of fire, 

 frost, drought, an unfavourable condition of the soil or the 

 attacks of scale insects. 



158. In some cases the direct One Factor 



connection between the injurious factor at work and the diseased alc " ie ls 

 condition of the plant produced by it is obvious. We may, for p^n/biiTfor 

 example, see a larva directly feeding on and destroying the a Disease. 

 leaves, or we may notice that the death of our tree follows 

 immediately after an unusually severe frost, and in such cases it 



is natural to conclude that in the absence of the larva or frost the 

 disease would have been avoided. At the same time from what 

 has been said above regarding the struggle for existence it will 

 be realized that one factor alone is rarely responsible for a 

 disease and this may frequently be the case even in such appar- 

 ently obvious examples as the above. The destructive larva, 

 for instance, might not have harmed our trees if the weather 

 or some other factor had not favoured its development to an 

 unusual extent, the frost might not have injured our trees if 

 they had not been growing in a damp valley, their twigs in 

 consequence having remained soft and full of watery sap in- 

 stead of becoming lignified and matured like those of the trees 

 on the hills around. The discovery of one obviously injurious 

 agent at work is thus by no means necessarily a satisfactory 

 conclusion to the investigation of any particular disease. In 

 cases where we have not ourselves been able to note the first 

 appearance of a disease, it is of course most important to 

 ascertain, as far as possible, the character of the first visible 

 symptoms, the time when they were seen, and whether the 

 existen.ce of any unusual external influence, such as severe 

 drought, the occurrence of a fire, etc., was noticed when the 

 disease first appeared. 



