INSECTS AND DISEASES The ailments of plants are constantly becoming more numerous and the knowledge con nected with them more complex, owing to dissemination of the parasites into new regions, the increase of food supply due to new and more extended cultures, to changes in habits of parasites and hosts consequent on the disturbance of the normal equilibrium in nature. At the same time, however, the means of contending with these difficulties are increasing with phe nomenal rapidity. Numbers of persons are now employed at public expense in the study of insects and diseases and in devising means of combating them. This is the guarantee of the future. In fact, our present-day agriculture would be impossible were it not for the entomologists and plant-patholo 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 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 commonly dated from the work of M. J. Berkeley (1803-1889) 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 gress in contention with in sects is to be looked for.
The gradual growth of the idea that one plant may he 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 diseases are yet known only by their external manifestations. Of these, peach-yellows and other peach diseases are examples. What causes the mal-nutrition and what carries the disease are undetermined. No doubt many ailments of plants are physiological and organic,—using these words in their human-medicine sense—rather than due to germs or filamentous fungi. Plants have scarcely begun to be studied in respect to their intimate pathological processes and their response .to sanitary or unsanitary environment. Very likely we await a new era in plant cultivation.
The plant diseases that are likely to be most clearly recognized by the general observer are those occasioned by the filamentous fungi. These low spore-bearing plants are related to the molds that appear on bread and decaying substances. It is impossible for one who has not studied these forms patiently under a microscope really to understand what they are. The ragged and spidery pictures of them that appear in the public prints convey little intelligence to the general reader. Perhaps Figs. 5G, 57 and 58 will help to a vague understanding of what these parasitic fungi are, and how they work. These fungi are species of plants, without flowers or seeds or leaf-green or any of the parts and organs that we ally associate with plant forms. They have neither roots ;:nor leaves, for they do not abstract mineral food from the soil nor construct organic food ,44,i in the presence of sunlight. They #,,, live on organic compounds,—that is on foods that :':„ have already been formed or organized by other plants or, through them, by ani- t:''.' '?1:' ;'ie, mals. They run their vegetative root-like threads, or mycelium, into the food supply; ,,,,,,,,;,,,, ;. 1 and they propagate their kind by means of special ized cells known as spores. The injury they do to their host is of two kinds,—they appropriate food, and they impair the tissues by punc turing them or breaking them down and by plugging the vessels or natural open ings.