150 PLANT PHYSIOLOGY 



would then be the greatest demand for carbon-dioxide to be 

 elaborated into food, and the gateways of the intercellular 

 passages should be open to aUow the free entrance of car- 

 bon-dioxide. If the stomata close in bright light to guard 

 against excessive transpiration, food production diminishes 

 greatly. The closing of stomata more or less completely 

 in darkness, the decrease in the size of the openings as the 

 light diminishes, may be coupled with these facts : the pro- 

 portion of oxygen in the air is so much larger than that of 

 carbon-dioxide, and the rate of respiration so much lower 

 than that of photosynthesis (see p. 65), that the stomata 

 may well decrease in size, or even close for a time, with- 

 out interfering with respiration ; transpiration on dry nights 

 might be excessive or at all events would tend to cause in 

 the cells an accumulation of salts unnecessary at the time 

 in amount and kinds; the leaf is cooled by transpiration, 

 and loss of heat by this means might be undesirable; in 

 nyctitropic plants, and in others habitually living where the 

 air is damp, the closing of stomata in darkness is less com- 

 mon than in plants growing under more ordinary condi- 

 tions. Stomata which have remained closed during the 

 night, begin to open at daylight in the morning. Heat tends 

 to open the stomata. This may have injurious or fatal re- 

 sults. If the leaves of an evergreen, for example of hoUj^, 

 are warmed by air and sun while the roots are encased in soil 

 so cold and dry that the root-hairs absorb too little water, 

 the opening of the stomata is very likely to be followed by 

 excessive transpiration, by the drying out and death of the 

 plant. Indeed, most cases of "winter-killing" are to be at- 

 tributed to the inability of the plant to balance transpira- 

 tion by absorption, rather than to actual freezing to death. 

 The majority of plants close their stomata when their 

 leaves are wet. This can be demonstrated by putting in 

 water strips of epidermis from the leaves. The advantage 

 is more obvious than the means by which it is attained. If 

 the stomata are the entrances of the passages through 

 which the necessary exchange of gases and vapor takes 

 place, these entrances must be kept free from whatever 

 would hinder the exchange when the stomata are open. 



