200 REPORT OF THE COMMISSIONER OF AGRICULTURE. 
house has reached a higher degree than is quite desirable before venti- 
lation can be given, and when the ventilators are opened a sudden iow- 
ering of the temperature takes place. Such rapid changes are specially 
provocative of mildew, which will be found to make its appearance upon 
the leaves contiguous to the openmg. When these conditions occur— 
and they are unavoidable in ordinary management—the temperature can 
be lowered several degrees by sprinkling the plants and floor of the 
house with water, after which the ventilation may be given gradually ; 
the moisture in the air will tend to modify any injurious effects from the 
ingress of the cooler external air. These sudden changes of tempera- 
ture are acertain cause of mildew on young, tender leaves. Their effects 
are frequently seen on the leaves of peach trees in early summer, cans- 
ing them to blister and producing the appearance commonly called “curl” 
in the leaves, and which is frequently, but erroneously, supposed to be 
caused by the aphis, which is sometimes, but not always, present. 
Rose-growers know that the utmost care in ventilation will not al- 
ways enable them to prevent the young leaves of roses from attacks of 
mildew. In all such cases a saturated atmosphere is beneficial. 
ROTATION IN CROPPING. 
It may be surmised that the necessity for rotation of crops soon be- 
came apparent to the earlier cultivators. They would discover that 
their best efforts in appliances were unavailable in maintaining a contin- 
uous profitable growth of the same kind of plant on the same soil. When 
soils became unproductive it was supposed that the land required rest, 
hence the practice of fallowing was introduced. Fallowing was a com- 
mon practice among the Romans. It was their usual course to allow 
the land to rest after each crop—a crop and a year’s fallow succeeding 
each other. Where manure was applied two crops were taken, and ou 
some lands several crops were taken between the fallowing periods. 
It was a very natural deduction that the land required rest when ob- 
servation showed that after successive crops of the same plant, it refused 
to grow, although the land had not apparently diminished in fertility. 
The agriculture of the ancient Egyptians being confined to the banks 
and lowlands adjacent to rivers, where from annual overflows a rich de- 
posit of mud and sand was left on the surface, which formed an annual 
layer of fresh material, did not include the process of fallowing or rest- 
ing lands, because constant fertility was maintained by the annual top- 
dressing which was left by the receding waters. 
The practice of resting and fallowing soils, or that of changing the 
crops more or less systematically, has always been found to be advan- 
tageous, although the reasons for its necessity have not been satisfac- 
torily explained. 
Various theories have been offered by physiologists explanatory of the 
principles upon which the benefits of rotative cropping depends. Modern 
chemistry has shown that plants require certain mineral substances for 
their support, and that although the same primary elements may be found 
in all, yet they are found to be in very different proportions in different 
kinds, some showing a mere trace of a substance which may abound in 
others. These mineral matters being obtained from the soil, it follows 
that if they are not present in sufficient quantities or do not exist in a 
sufficiently soluble state so as to be taken up by the roots, the plant 
which demands them for its normal growth must suffer in consequence 
of such deficiency; and in regard to specific inorganic substances, it is 
evident that the plant which requires a large percentage of such would 
