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Rh was a good key for the determination of species while the number was still small, but it was far from being a natural system of plants. In order to attain to such a system, one had to dig deep into the development and the inner structure of plants. This permeating of systematic botany with general botanical knowledge raised this study to a height where it might with propriety be called the earlier systematic botany.

The separation of plant species proceeded, therefore, no longer, as it did earlier, upon the basis of external characters, but came to be more and more promoted through the facts furnished by anatomy and embryology. That pure physiological characters, i.e., characters that find expression in the life processes of the plant, should be brought forward to distinguish species is one of the latest discoveries. A physiological character of plants would formerly have been held as unreal. Distinguishing characters were wanted which were always to be found in dead material, such as lies in our herbaria. So long as that sort of character sufficed there was nothing to be said against the proceeding. Now, however, we meet plant forms whose scientific nature is to be recognized only in their life activities. A Swedish botanist has made the observation that rust fungi exist which on morphological characters are impossible of separation, but are characterized only in this, that they will live on one or a few species of grasses, but will not develop when transferred to other grasses which are hosts for fungi of exact morphological equivalence. The well-known black rust of grain (Puccinia graminis) occurs upon wheat, rye, oats, barley, and several uncultivated grasses. It was formerly supposed that the grain rust could choose at will between these species of grasses. This is, however, not the case. It is known, for example, that the rust of rye can develop on barley, but not on wheat and oats, and it is evident that several physiological forms of grain rust may be distinguished upon this ground.

So in the progress of research has come about a union between two branches of botany which appeared widely separated, so widely that it was formerly supposed that the chasm between them would never be bridged over, i.e., between systematic botany and physiology in its broadest sense—indeed physiology in the narrowest sense of the doctrine of function. It is plain that all other fields of botany stand in reciprocal relation with physiology, but it required a long time for this state of things to come about.

Nothing would seem more natural than that in scientific investigation a plant form and the function of its organs would be equally considered—to consider it as a machine, whose parts are arranged for a purpose and in their combined action accomplish an intended result.

One need not wonder, therefore, that investigations undertaken at an earlier time, with the purpose of making clear the agreement between form and function of the plant organ, wholly miscarried and led to vague speculations and barren telleology. It was in the midst of our