6 U. S. DEPARTMENT OF AGRICULTURE. 



loss of water vapor by the plant through its stomata,- or 

 transpiration, and by evaporation from cut surfaces. The 

 loss of water by plant organs is illustrated very strikingly 

 by the phenomenon commonly known as "sweating," which 

 is the result of condensation of this water vapor. 



The first result of excessive water loss is known as wilting. 

 Wilting is especially frequent in leafy crops with large 

 transpiring surfaces, such as celery, lettuce, or spinach. The 

 compact, heavy crops such as deciduous and citrus fruits, 

 potatoes, and onions are made up of storage tissues and are 

 much less subject to water loss than the more leafy ones. 

 They lose water much more slowly because of their rather 

 thick protective skin and fewer stomata. They also have a 

 greater bulk of water available in their tissues, and a larger 

 volume of tissue relative to their transpiring surface. 



Drying of the surfaces of plants, with a slight wilting of 

 leafy plants, before they are shipped or stored is advisable. 

 Wet plant surfaces favor the development of bacteria and 

 fungi and, when stomata are filled with water, easy ingress 

 is provided for bacteria. Many crops are sweated, cured, 

 or kiln-dried before they are put into storage or shipped, 

 to avoid undue shrinkage by loss of water, and to dry and 

 harden the tissues to render them less subject to attack by 

 bacteria or fungi which flourish on tender tissues under 

 moist conditions. Continued loss of water by plant tissues 

 soon results in their death after which they may become the 

 prey of bacteria and fungi. 



i Not all of the decrease in weight of fruits and vegetables 

 in storage is due to loss of water. A thoroughly cured sweet 

 potato loses weight and shrinks in storage even when stored 

 in a very humid atmosphere, especially if the temperature 

 is high. A part of this loss is due to transpiration, the 

 remainder to respiration. At low temperatures, the pro- 

 cesses go on very slowly, but ultimately lead to physiological 

 breakdown. In other words, there is a time limit to the 

 storage of all living plant parts. The period is especially 

 brief in the case of succulent crops. Each crop has its period 

 beyond which it is not advisable to store it. For example, 

 a Jonathan apple cannot be stored as long as a Winesap. 



Proper aeration and refrigeration are essential in storage 

 places. The former prevents the accumulation of water 

 vapor and attendant sweating, provides oxygen, and re- 

 moves carbon dioxide. The latter decreases the rate of 

 digestion and respiration in the plant tissue, thus lowering 

 the rate at which the latter consumes itself. More impor- 

 tant, however, is the effect of these factors upon the bacteria 

 and fungi which, because of their power to cause decay, are 

 the chief enemies of stored fruits and vegetables. Bacteria 

 and fungi cannot attack plants when the plant surfaces are 

 dry. Furthermore, at low temperatures their activity is so 

 reduced that they seldom attack plant tissues. 



