SYMPTOMS OF DISEASE. 187 



fungi, for instance, induce different coloured spots 

 according to the age of the leaf or other organ 

 attacked, or according to the species of host, the 

 weather, etc. Moreover the spots due to these 

 parasites are frequently yellow when young and 

 some other colour, especially brown or black, 

 when older. 



Scale is the name given to the characteristic 

 shield-like insects {Mytilaspis, Aspidiotus, etc.) 

 which attach themselves to branches of Apples, 

 Pears, Oranges, Camellias, and numerous other 

 plants, and suck the juices. It is the female 

 insect which has the body broadened out into the 

 " scale," under which the young are brought up. 

 Enormous damage has been done by some forms 

 e.g. the San Jos^ scale in the United States. 



The superficial resemblances of the patches of 

 eggs of some Lepidoptera to Aecidia and other 

 fungi may be noted in passing e.g. Boinbyx 

 neustria on Apple twigs, Aporia Crataegi. 



White ox greyish spots are the common symptom 

 marking the presence of many Peronosporeae and 

 Erysipheae in or on leaves, e.g. Peronospora Tri- 

 foliorum^ P. parasitica on Crucifers, etc., and 

 SphaerotJieca on Hops; also Septoria piricola, Cysto- 

 pus, Entyloma Ranunculi., etc. 



White spots are also caused by insects such as 

 Tetranychus (red spider) on Clover and other 

 plants. 



Yellow, or Orange-coloured Spots. In cases 

 where these occur on leaves, and in the case of 

 grasses, etc., on the leaf sheaths as well, they 



