134 PLANT PHYSIOLOGY 



are passed over with the superficial explanation that they are the 

 result of enzyme action, and, therefore, require no further considera- 

 tion. It is known that the ferments are largely concerned with the 

 regulatory production or modification of numerous metabolic pro- 

 ducts. The activity of each enzyme is circumscribed, yet the power 

 to do work borders upon the miraculous. It is asserted that invertin 

 may invert 100,000 times its volume of cane-sugar, and pepsin may 

 transform 800,000 times its volume of proteids. The chemist is 

 especially concerned with the composition and occurrence of these, 

 but the physiologist is interested not alone in the occurrence and 

 specific action of the enzymes, but also with the effects upon the 

 general metabolism of the individual plant, with the methods and 

 conditions regulating the secretion of these products, and with their 

 vitalities or limiting external conditions. Ferments may be concerned 

 with external cellular digestion, that is, with the solution and absorp- 

 tion of foodstuffs from without, thus necessitating exosmosis, or 

 with intracellular modifications, preparatory to the direct use of the 

 substances modified in metabolism or in translocation. Again, the 

 ferments may be present only at a certain definite period in the life 

 of a cell, produced, undoubtedly, by special requirements and special 

 stimulation. 



When isolated, or at least when outside of the cell, many enzymes 

 are most active at temperatures far above those which may be main- 

 tained within the living cell. An explanation of this fact is difficult. 

 Comparative studies of their reactions to light, heat, toxic agents, 

 and other stimuli should be made. In the penetration of parasites, 

 cellulose-dissolving ferments are important, but further information 

 is needed before it can be said that the presence or absence of such 

 enzymes to any great extent affects the resistance of certain varieties 

 and species to fungous attacks. It has been stated that the resistance 

 of plants to fungous attacks is due largely to the presence of certain 

 enzymes or toxalbumens present in the cells of the host; and by 

 others it has been suggested that susceptibility is frequently a special 

 property due to the presence of certain oxidases, which are regulated 

 by external conditions. 



It has been shown that the mosaic disease of tobacco and other 

 similar diseases are accompanied by certain oxidase ferments which 

 appear to prevent the digestion of reserve food. The ferment is de- 

 veloped in the growing parts of the plant, it may be transferred 

 from plant to plant, and on the decay of the diseased organism, it is 

 supposed to be set free in the soil. It is believed that it is then capable 

 of diosmosis and infection of the young seedling. While it cannot 

 be shown at present that the enzyme is beyond all question the 

 direct cause of the disease, this field of work is certainly one which 

 might yield most interesting results. In this connection it may be 



