DISEASED CONDITION OF PLANTS 43 
veins and adjacent tissues. This is easily noticeable 
from the upper surface of the leaf, but an examination 
of the lower surface reveals a peculiar powdery appear- 
ance on the veins. These are seen to be much swollen, 
and in places the swollen tissues have become ruptured. 
Similar pimples appear on the lamina above small veins. 
The lesions are seen to best advantage on the older leaves, 
and show as translucent areas when held up to the 
light. In bad cases the spots dry up and tiny holes are 
produced in the leaves. The lesions on the geranium 
become corky, and in bad cases the leaves and petioles 
are covered with corky spots and ridges. Microscopic 
examination shows that the cells have become much 
swollen and distorted, and the chloroplasts are few in 
number—the excessive enlargement of cells causing the 
tissues literally to burst. The disease is favoured by 
poor light, a high soil temperature, and over-watering. 
Frequently the plant recovers when removed out of doors, 
and control may be effected by lowering the soil tempera- 
ture, reducing the water supply, and providing abundant 
light and ventilation. 
The Soil 
The intimate relationship between soil conditions and 
the growth and health of plants has been apparent to 
growers from the earliest times, and successful cultivation 
largely depends upon a knowledge of the effect of the 
many factors which influence the conditions operative 
about the plant roots. Many excellent treatises have 
been written about the soil, and as these are available 
to every one there is no need to discuss the problem in 
any but the briefest way. 
The physical conditions, chemical conditions, and 
vast population of microscopic animals and plants in 
the soil influence its fertility and freedom from disease, 
and therefore are important factors in crop production 
and health. 
