. Cyclopedia of farm crops. Farm produce; Agriculture. S6 INSECTS AND DISEASES dm gists. These persons have become as much a part of our modern needs as, in a related realm, have the physicians and sanitarians. For the most part, the work of insects is at once recognizable ; but plant diseases are obscure as to cause, and it is only within the past fifty B years that very careful study has been made of them. The special study of para- sitic fungi, which cause many of the dis- eases of plants, is cemmonly dated from the work of M. J. Berkeley (1803-1§89) in Eng- land about the middle of the cen
. Cyclopedia of farm crops. Farm produce; Agriculture. S6 INSECTS AND DISEASES dm gists. These persons have become as much a part of our modern needs as, in a related realm, have the physicians and sanitarians. For the most part, the work of insects is at once recognizable ; but plant diseases are obscure as to cause, and it is only within the past fifty B years that very careful study has been made of them. The special study of para- sitic fungi, which cause many of the dis- eases of plants, is cemmonly dated from the work of M. J. Berkeley (1803-1§89) in Eng- land about the middle of the century just passed. It is also astonishing that the life- histories of most of the common insects were not understood a century ago ; and there are numerous insects all about us whose life-cycles have never been worked out. A good part of our current, economic ento- mological study is devoted to discovering the main phases of the insects rather than to the adding of new facts and incidents. The subject of the intimate relationship of insects to each other, to weather, to food supplies, and to other factors of their environment, where- by their relative prevalence is largely determined, is yet practically an unexplored field ; yet it is in this eco- logical domain, rather than in merely destroying in- sects by what may be called mechanical means, that the greatest permanent pro- A gress in contention with in- M, sects is to be looked for. /«> The gradual growth of \%@ '*!2 the idea that one plant may be parasitic on another and cause what may be called a disease, would be a subject of great attractiveness to one who is interested in human history. The idea is so recent that it should not be difficult to trace. A recent development of it is the discovery that there are germ diseases of plants as well as of animals, a history that is recorded by its literature in E. F. Smith's "Bacteria in Relation to Plant Diseases" (Carnegie Institution, 1905). Other classes of dise
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