172 Principles of Plant Culture. 
335. The Cultivator will often Need to Consult the 
Specialist in dealing with fungous diseases. In many 
cases, it will be difficult or impossible for him to decide as 
to the exact nature of a given trouble without careful train- 
ing, and skill in the use of the compound microscope. Spe- 
cialists in this line are now employed by the governments 
of most civilized nations, and by many agricultural experi- 
ments stations, and they should be freely consulted. Much 
may be learned, however, by studying the best books on the 
subject. The cultivator should learn to recognize the prin- 
cipal fungous diseases. 
Section VII. Toe Piant as AFFECTED BY WEEDS 
336. Weeds are plants that, while not parasitic, persist 
in growing where they are not wanted. They injure the 
desirable plants about which they grow, by robbing them of 
light, moisture and food, and their presence is an evidence 
of slovenly culture. The remarkable vigor and prolificacy 
possessed by many weeds enable them to overcome most 
cultivated plants, which they would soon subdue but for the 
aid of the cultivator. As with harmful insects and fungi, 
prompt and persistent efforts are most essential to the con- 
trol of weeds in cultivated grounds. 
337. Annual, Biennial and Perennial Weeds. With 
reference to their term of life, weeds and other plants, are 
divisible into three classes, viz.: annual, those that live but 
one season; biennial, those that live only two seasons; and 
perennial, those that live an indefinite number of seasons. 
Those of the first class usually seed most abundantly, and 
hence they are most widely distributed and appear in culti- 
vated grounds in the greatest numbers. Those of the third 
