446 PLANT PHYSIOLOGY 



during the time of storage when there can be no more influx of 

 carbohydrates. 



When storage is prolonged, processes of overripening and death 

 occur in the cells of the flesh, the activity of proteolytic enzymes 

 increases, and autolysis begins. The connection between the 

 cells is lost, and the flesh is transformed into a soft pulp. Such 

 overripened fruits are readily attacked by molds and bacteria. 



The transformation of starch into sugar and the oxidation of 

 tannins and organic acids, which proceed slowly in the case of 

 natural ripening, may be accelerated artificially by means of 

 special stimulation. The method of treating mature unripe fruit, 

 chiefly bananas and tomatoes, with ethylene gas, elaborated in 

 America by Harvey, has obtained wide practical apphcation. 

 The treatment is carried out during 2 to 3 days in tightly closed 

 chambers containing an insignificant amount of ethylene, about 

 1 part of ethylene per 1,000 parts of air. The effect of ethylene 

 is explained by the fact that it hastens enzyme action and may 

 increase the permeability of the protoplasm and thus faciUtate 

 the access of oxygen. This increases the oxidation processes 

 leading to a more rapid transformation of tannins and organic 

 acids. An increase of permeabihty also promotes the diffusion 

 and activity of the enzymes, this Hkewise contributing to the 

 digestion of starch and other higher carbohydrates and of the 

 pectinous substances that hold the cells together and thus leading 

 to a softening of the fruit. Besides the addition of ethylene, it 

 is necessary to keep the temperature at about 18 to 21 °C. and 

 the relative humidity of the air not below 90 to 95 per cent. 

 The same effect is produced on fruit by propylene and to a 

 considerable degree by acetylene. 



Artificial ripening of the fruit differs from the natural process 

 in that the seeds do not accelerate their development. The 

 changes occur principally in the cells of the flesh of the fruit. 



In ripe fruit, the seeds that are also ripe and consequently 

 ready to germinate are frequently surrounded by a watery 

 medium, and the question arises as to why germination is not 

 observed inside the fruit. The absence of germination may be 

 determined by various causes. In many succulent fruits, e.g., 

 in apples, pears, and especially in hawthorn berries, the seeds of 

 even completely ripe fruit have not quite finished all the processes 

 preparatory to germination and require a rather prolonged period 



