RELATION OF PLANT PHYSIOLOGY TO OTHER SGIENCES. 431 



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 devel- 

 opment and the inner structure of plants. This permeating of sys- 

 tematic 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 embry- 

 ology. 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. Distin- 

 guishing 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 (Puc- 

 cinia graminis) occurs upon wheat, rye, oats, barley, and several uncul- 

 tivated 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 doc- 

 trine 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 con- 

 sidered — 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 



