The study of the relations between weather and crops is really a branch of a science of broader scope, known as _phenology_. This science is devoted to the investigation of all periodic phenomena of plant and animal life that are controlled by the weather. There are, in some parts of the world, large corps of phenological observers, who maintain records year after year of the leafing, flowering, and fruiting of both wild and cultivated plants, the migrations and first songs of birds, and various other events of a biological character that recur with the seasons. In the course of time it becomes possible to compute from such records the normal dates of these events; and then, in any particular year, a comparison between the actual dates and the normal shows whether the season is early or late, and by how many days. Phenological observations on plants also make it possible to draw charts showing the normal march of the seasons over a country, expressed in terms of plant life, and such charts are often more valuable to the agriculturist or horticulturist as a guide in selecting varieties for cultivation and in timing his operations, than any charts that can be compiled from ordinary climatic data. Some admirable charts of this kind have been drawn for parts of Europe. There are many practical applications of phenology to agriculture, and there would be more if phenological observations had been made more extensively throughout the world. Good phenological charts of different regions would, for example, greatly facilitate the work of foreign plant introduction carried on by the United States Bureau of Plant Industry. In the United States phenological observations were made systematically between 1850 and 1863, but only desultory work has been done in this line subsequently. The most comprehensive individual record is that maintained by Thomas Mikesell from 1873 to 1912, at Wauseon, Ohio, and published in full by the Weather Bureau in 1915. The old rule of American farmers, inherited from the Indians, that the time to plant corn is when the leaf of the white oak is “the size of a mouse’s ear,” illustrates the use that can be made of so-called “index plants” of the native flora as guides for farming operations. Professor A. D. Hopkins writes on this subject: