The study of structure, whether macroscopic or microscopic, leads one naturally to investigate the functions of the parts. The study of functions is physiology, and since we have given up the older notions as to the sacredness, the supernaturalness, of the phenomena of life in favor of the more rational view that they are chemical and physical, all physiologists to-day are pressing forward, with chemistry and physics as their allies, to larger knowledge and clearer ideas as to what constitutes life. Far as we still are from a solution of the riddle of the ages, yet during the present century progress hitherto unequaled has been made. Animal and vegetable physiologists are now going hand in hand toward their common goal. In studying the processes of nutrition, growth, reproduction, and the phenomena of perception, reaction, and exhaustion, they are supplementing one another. There are indeed some few physiologists of training and disposition so broad that they decline to be known as animal or as vegetable physiologists, but wish to be called what they really are, students of the functions of living organisms and seekers after light from whatever source upon life itself. The more one studies the physiology of animals and plants the more one sees that the distinctions which have been made between the two are more apparent than real, and that as in so many other cases our names are for convenience rather than for the exact expression of the truth.
The physiologist finds that there are two great classes of plants: (1) Those which, able to obtain from the crude materials of the soil and the air all the elements which they need for their nutrition, lead self-dependent existences; and (2) those which, unable to elaborate their food from such matters, must get it from other organisms, either directly or indirectly. All animals depend ultimately for their food upon those plants which are able to elaborate living matter from lifeless mineral salts, water, and air. But there are quite as many plants which are as dependent as animals. The groups of parasites, the flowering and the flowerless, the dodders and many of the fungi and bacteria, for example, are absolutely dependent upon living organisms, either animal or vegetable as the case may be, for their food. Other plants extract from the dead and more or less decayed remains of organisms the highly elaborated nitrogenous and carbon compounds which are essential to all life. Still others are fairly independent, but supplement their self-made food by other means; for example, the whole group of insectivorous plants and several of the orchids. The saprophytic plants, those living on dead organic matter, are very important in the economy of Nature. They accomplish rapidly, and with much less damage or offense to other organisms, the decomposition of otherwise waste matters which
