66 DISEASES OF CULTIVATED PLANTS 



not treated with ice or drops of water, remained free from 

 ' spot.' The disease is produced most readily on plants that 

 have been liberally supplied with water, and grown in a high 

 temperature. The same kind of orchid, when in a resting 

 condition, scantily supplied with water, and kept in a low 

 temperature, could not be induced to ' spot.' 



So far as cultivated orchids are concerned, it may be stated 

 that spot is favoured by (i) too high a temperature; and 

 (2) by a too liberal supply of water. 



The actual cause of spot is due to watering or spraying 

 with a falling instead of a rising temperature. 



Massee, Ann. Bot.^ g, p. 421. 



Silver-leaf. Plum-trees suffer most, but peach, apricot, 

 sloe, and other members of the Pruneae also show the disease. 

 The foliage is the part affected, the upper surface of the leaf 

 losing its normal green colour, and assuming a silvery sheen, 

 hence the name ' silver-leaf.' This appearance is due to the 

 presence of air-cavities under the cuticle, formed by the more 

 or less complete separation of the cells of the epidermis, due 

 to the splitting apart of the vertical walls. In other respects 

 an affected leaf appears to be perfectly normal, there is no 

 stunting, and the chlorophyll is present as usual. In some 

 instances a single branch is attacked, which usually dies during 

 the second season after showing the disease, the rest of the 

 tree remaining perfectly healthy. This is more especially the 

 case with trees grown under glass. When growing in the 

 open the whole tree is not infrequently attacked, and then 

 usually succumbs within two or three years, in some instances, 

 however, lasting much longer. Trees when once attacked as 

 a rule do not recover. The disease appears to be much more 

 prevalent at the present day than in past times, or perhaps it 

 is more frequently noted now. 



Professor Percival claims to have proved that a fungus 

 (Stereum purpureum] is the primary cause of the disease. 

 This, however, I have not been able to confirm ; I have 

 examined hundreds of diseased trees without finding a trace 

 of the fungus, either on the surface or in the tissues. 



Another view is that the injury is due to excess of nitro- 

 genous matter in the soil, but the disease often breaks out on 

 a large scale in places where no such excess of nitrogenous 

 matter exists ; besides, this idea could not account for the 



