DISEASES OF PLANTS, 349 
In such case plants may, sometimes recover by | 
change of soil. 
§ 320. | 
Icterus differs from chlorosis,’ only in its colour 
and by its cause, which is cold coming on early in 
autumn. It is indeed the natural death of the 
leaves, and may only hurt the plant itself, when the 
cold begins in autumn before due time. 
| §, 32). 
Arisa ca, or dropsy, arises in plants from long 
continued rain, or too profuse watering. Single 
parts in this case become preternaturally swelled, 
and commonly putrify. Some of the bulbous and 
tuberous roots, for instance, are often greatly swelled 
after rain. Fruits lose their taste, and become wa- 
tery. Seeds do not get ripe or the plant pushes out 
young shoots unseasonably from the stem. Most of 
the succulent plants suffer from too copious a supply 
of water. 
Anasarca in plants is generally incurable. 
R 399, | 
Phthiriasis is that disease of plants, cae the whole 
of it is covered with small insects, which feed on 
its sap, disturb the important operation .of transpira- 
tion, and of course hinder the future’ evolution of 
its parts. ‘This disease is produced by three differ. 
ent species of insects. In the first place, by the 
aphis, of which each plant has almost a peculiar 
species 
