YSIOLOGY. Nothing was known of the life of plants in the early part of the Seventeenth Century except what had been learned through agricultural and horticultural operations. It was in the latter half of the Seventeenth Cen tury that the science of Plant Physiology was founded, long after the physiological sig nificance of the different organs of the human body and of most animals was generally known. The history of the gradual discovery of the various functions of plant-organs is long and full of interest. The foundation of all physiology is the direct observation of vital phenomena, and these must be evoked or altered by experi ment. It is by means of this experimentation that a definite knowledge of physiological pro cesses has developed, until now the student of plant physiology is in possession of a vast body of facts. Vital phenomena in plants are essen tially the same as in animals, but are often simpler; and hence plants frequently furnish the clues for the interpretation of the more oomplex activities of animals.
The important general functions of plants, which furnish the subject-matter for physiologi cal research, are as follows: Absorption of ma terial and of energy from the outside world; transfer of water through the plant-body, by which materials are properly distributed; trans piration. by means of which water is lost from
the plant's surface; nutrition, including photo synthesis (the manufaeture of carbohydrates), digestion (the conversion of foods into soluble form for transfer). and assimilation (the organ ization of protoplasm from food-material) ; se cretion; respiration. by means of which energy is liberated for the activities of the body; growth ; and movement, which includes irritable responses to many stimuli.
For example. in the higher plants the root is an absorbing organ for water and soluble soil substances; in the root. stein, and leaves there are certain sets of vessels along which water and food-materials travel readily; the leaves and the young stem surface are organs for the absorption and evolution of the carbon dioxide and oxygen, they also give off water by evaporation (transpiration), and are most im portant organs for the manufacture of carbohy drate foods. Although these various processes are distributed among the organs of a complex plant, they may all go on in a single cell of the simplest plant.