PLANT PHYSIOLOGY PRESENT PROBLEMS 135 



stated that peach "yellows" and several other important contagious 

 diseases are believed to be of somewhat similar nature. It is also 

 claimed that the keeping qualities of fruits may bear a certain relation 

 to the amount of enzymes present at the time of storage; and, there- 

 fore, a knowledge of the time and conditions of the production of 

 such enzymes would have great economic value. 



In general, Czapek found no enzymes to occur in the excretions 

 from the roots of higher plants, and it is now generally believed that 

 the roots of one plant may develop no excretions injurious to neigh- 

 boring plants, and, therefore, there may be no biological relation 

 between the roots of non-parasitic plants associated in the given 

 plant society. It must be said, however, that the information at 

 hand may not be taken as final. There are yet some peculiar facts 

 with relation to the rotation of crops which may not be readily 

 explained on the grounds of the exhaustion of plant nutrients or of 

 the physical condition of the soil. 



The fermentation of tobacco and tea, or hay and manure, involves 

 enzyme actions which in recent times have received some attention, 

 although the problems which are of most physiological importance 

 require solution. The general belief is that in all cases of enzyme 

 action these compounds do not form a part of the substance upon 

 which their action is exerted, but they act as a key in each particular 

 case, unlocking, or rendering labile, a certain organic compound, 

 which is then subject to rearrangement and transformation. This 

 is all, however, too speculative for profitable consideration, although 

 such speculation may have no evil influence if it is not permitted 

 to encourage the reference of all unusual phenomena to an unusually 

 obscure and difficult process. 



The early perfection of water-culture methods permitted a careful 

 study of the mineral nutrient requirements in the higher plants. 

 Pure culture methods have afforded a more accurate means of study- 

 ing the needs of fungi and certain algae. As usually installed, water 

 cultures of the higher plants contain bacteria, so that they afford 

 only a practical test of the requirements. The problem demands 

 some confirmatory tests, at least, under pure culture conditions, 

 particularly when organic compounds are employed. It is possible 

 to grow, in a limited way, higher plants under pure culture conditions. 



With the fungi, exact studies may be made upon the influence 

 of the different nutrients on the general form and upon the produc- 

 tion of conidia, etc. It has been found, for instance, that, in the 

 absence of potassium, Sterigmatocytis niger may produce no conidia 

 or very curious modifications of the conidiophores. By far the most 

 interesting problems with relation to the mineral nutrients are those 

 which have to do with the roles of these elements in metabolism. 

 The effect of the lack of one or another element is made manifest by 



