22 The Principles of Fruit-growing. 
plant or the part affected. Such troubles are peach 
yellows, and black-knot of the plum and cherry. A 
law cannot be enforced unless public sentiment 1s 
behind it, and when public sentiment is aroused the 
law is not needed. Yet a law is often useful for a 
time to awaken publie sentiment and to call attention 
to the evil. The final recourse is always greater 
knowledge and enlightenment. 
' There are also insurmountable difficulties in the 
enforcement of laws designed to control the spread 
of noxious insects and fungi, because it is practically 
impossible to detect the eggs of insects or spores 
of fungi upon a large number of plants, and because 
there are so many natural and uncontrollable ways 
in which the parasites may spread. The original 
Maryland law, designed to prevent the introduction of 
fruit-tree diseases and pests, was a case in point. 
It required that “whenever any trees, plants or vines 
are shipped into this state from another state, every 
package thereof shall be plainly labeled on the out- 
side with the name of the consignor, and a certifi- 
eate showing that the contents had been inspected 
by a State or Government officer, and that the trees, 
plants or vines therein contained are free from all 
San José scale, yellows, rosette and other injurious 
insect or disease.” It would be impossible for any 
botanist to certify that a dormant tree were free of 
all disease; and even in the matter of San José 
scale, an entomologist could not give a clean bill 
of health without giving more time to the examina- 
tion of a tree than it is worth. In the operating of 
