HORMONES, AUXIMONES, VI T AMINES, AND TOXINS 241 



These hormones, as a class, are chemical substances which have 

 very little attraction for, or power of combination with, water; 

 and it has been suggested that the ease with which they penetrate 

 the protoplasm is due to the fact that they are not held at the sur- 

 face by combination with the active water molecules which are 

 present in the surface layer. 



The principal effect which is supposed to be produced by 

 these " hormones " is the stimulation of the enzymic activity, 

 particularly that of the degenerative processes which take place 

 late in the plant's life, at the flowering or ripening periods. 

 Many of the changes which take place normally at ripening time, 

 such as the change in color from green to yellow or red and finally 

 to brown or black, when the fruit or vegetable is fully ripe, can be 

 greatly accelerated by treatment with these substances. Hormones 

 are similar in type to the ethereal salts, or esters, which constitute 

 the natural essential oils that develop in many plants at this 

 stage of their growth. Hence, it seems probable that these 

 changes in plants which are maturing naturally may be hastened 

 by the hormone action of the esters and similar bodies which are 

 developed in largest quantities at that stage. It has been pointed 

 out that the characteristic group which is present in many natural 

 glucosides is of the same general type as the " hormone " sub- 

 stances which are used in the artificial stimulation of the flowering 

 or ripening changes. This fact, together with the possibility of 

 the liberation of greater percentages of these aromatic compounds 

 from their glucoside combinations at the later periods of plant 

 growth, is assumed, by some plant physiologists, to account for 

 the change from synthetic to degenerative processes at this stage 

 of the plant's development. 



Further, it has been suggested that the autumnal coloration of 

 leaves, and their dropping from the stems of the plant, as well as 

 the ripening of seeds, is probably determined by the liberation in 

 the plant, at that stage of its growth, or as a result of changed 

 climatic conditions at that particular season of the year, of the 

 hormones which either initiate or hasten the special enzymic 

 changes which distinguish the degenerative from the synthetic 

 processes of the plant. 



Similarly, it has been suggested that parasitic fungi are able 

 to penetrate the host plant by first excreting " hormones " which 

 bring about degenerative changes in the tissues of the host plant 



