RESPIRATION OF PLANTS 95 



effects should be considered as irritations and are characterized 

 by an increase of respiration of a temporary nature that after a 

 short period of increased rate returns to the initial intensity. 



Not only chemical substances but also physical agents may act 

 to irritate or stimulate the cell. For instance, abrupt changes 

 in temperature, whether an increase or a decrease, may cause a 

 temporary speeding up of the process of respiration. The same 

 effect may be produced by a change of light intensity. A sharp 

 and prolonged stimulation is caused by mechanical injuries to 

 plant tissues, especially such as occur in the cutting of massive 

 organs into pieces. In this case, there is the additional effect 

 of the improvement of the conditions of gas exchange caused by 

 the increase of the surface of contact with the atmosphere. 

 Various deeply penetrating radiations, such as X rays, rays from 

 radioactive substances, as well as ionization of the air and other 

 electric influences, also stimulate respiration. 



In some cases, there may be observed the depressing effect of 

 external agents upon respiration. Extremely interesting and 

 of great practical importance is the influence of carbon dioxide. 

 When accumulated in considerable amounts, it decreases or 

 inhibits respiration, as well as the processes of growth closely 

 connected with respiration. Such an accumulation of carbon 

 dioxide may frequently be observed in seeds with hard seed 

 coats. This is probably the reason why they do not show signs 

 of germination when moistened until the seed coat is mechan- 

 ically injured. This explains the striking fact that seeds of many 

 weeds and other plants may remain for years in the moist soil, 

 without germinating and without losing their germination 

 capacity. Their embryos remain iru a state that may be com- 

 pared to a condition of profound anesthesia under the influence 

 of some anesthetic. Carbon dioxide gas may be considered as 

 belonging to the group of anesthetic substances. 



Recently this action of carbon dioxide is being utilized for the 

 preservation of fruits and vegetables. Placing them in an atmos- 

 phere containing carbon dioxide at regulated concentration in 

 a closed container inhibits the growth of certain pathogenic 

 fungi and bacteria that otherwise would attack the fruit. Like- 

 wise, this procedure inhibits to a considerable degree the processes 

 of senility and decomposition of the fruit itself. Thus their 

 storage life is considerably increased. 



