identities between plant and animal life? To-day we know that plants respire in the same sense and for the same purpose as animals; indeed, the forms of respiration are the same in both kingdoms. Besides ordinary respiration in which free oxygen is taken up, there is, in plants as in animals, a so-called intramolecular respiration, in which fixed oxygen in highly oxidized compounds serves to carry on respiration. The newer investigation has acquainted us in no equivocal way with the power of motion—yes, even with the sensibility of plants. Slow movements which are to be detected by change of position during growth are common in plant life, but even very lively movements such as are exhibited by swarming of certain reproductive cells (swarmspores and spermatozoids) occur frequently in the lower groups of plants. And shall one not speak of sensitiveness in plants when it is shown that external influences such as light, gravity, etc., act as an irritant which the plant receives, conducts to parts more or less distant and responds to by some definite movement or in general by some definite reaction?
The principle of the division of labor has worked here as elsewhere n the natural sciences, first separating and then bringing together. Plant physiology has gone its own way, as has also animal physiology, the one not concerning itself about the other; and only enlightened minds have first discerned the inner identities of both, and felt themselves compelled in the solution of fundamental problems to reach out for data into the apparently foreign territory of the other. Thus, one of the greatest animal physiologists of the new era, Ernst von Brucke, who once occupied this same place of honor, to which investigator we are indebted for three great fundamental contributions in the field of plant physiology.
When investigation in each of the two fields had yielded a rich fund of usable data and had placed them in an orderly arrangement, the union of the two—plant physiology and animal physiology—began. When one takes up a recent work on animal physiology he discovers with satisfaction that already much consideration is given to the facts and conclusions of plant physiology. Recently certain works upon general physiology attest the natural association into which animal and plant physiology have entered.
The relations of physics and chemistry to plant physiology lie so closely before us and are so well known that I need not here go into nearer details concerning them. But that both these great fields of investigation stand in reciprocal exchange with their younger sister, plant physiology, I will illustrate by a characteristic example. One of the foremost living plant physiologists investigated the working of osmotic force in the life of a plant. He soon had to learn that, however much the physiologists had contributed to the knowledge of this question, both in elementary and advanced works, it was not sufficient for his purpose, and thereupon it was thought necessary by him to
