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about the time the buds are opening in the spring. Plum-pocket 
and black-knot are both best controlled by pruning back the dis- 
eased branches. If the tree is badly infected with black-knot the 
whole plant should be cut down and burned. 
INnsEcT-GALLS. 
Insect-galls include those injuries by insects which result in © 
abnormal swellings of the stems or leaves or other parts of the 
plant (fig. 37). The adult insect deposits its eggs in the young 
parts of plants, and the sting of the adult or the hatching of the 
young or both together furnish the stimulus necessary to bring 
about this abnormal and often very conspicuous growt r 
their general appearance insect-galls are likely to be uruced with 
fungous diseases. Since these injuries are of insect origin their 
consideration belongs more properly to the entomologist. 
PARASITES WHICH ARE BENEFICIAL TO THE Host. 
We ordinarily think of a parasite as something which is harm- 
ful to the host on which it occurs. This is usually the case, but 
there are some parasites which are not harmful, and on the other 
hand are actually beneficial to the plants on which they live. We 
have an interesting example of this among the various members 
of the pea family. If we should remove a pea-vine or bean-plant 
from the ground we would find the roots often covered with 
numerous swellings resembling miniature potatoes. These swell- 
ings are caused by a parasite which in this case is one of the 
bacteria. 
Plants need nitrogen in order that they may grow. While 
nitrogen is present in great quantities in Bas air most plants are 
pounds as it exists in the soil. The nitrogen compounds in the 
soil become very much reduced by excessive cultivation, so that 
it is often impossible to produce a healthy crop, and soils are 
then said to be worn out. The bacteria which grow on the roots 
of various members of the pea family are able to fix free nitrogen 
from the air in such a way as to make it available for the host 
