Control of Transpiration 51 



other times, and since the continual ingress and egress of the air 

 which brings it would entail a steady drain of moisture from the 

 plant by evaporation, the breathing pores in the leaves are usu- 

 ally provided with a pair of guard cells, which are so constituted 

 that they may be opened and closed, and thus exclude nearly all 

 the air from the interior of the plant ; or, by partly closing 

 them, to vary the amount of air which may be admitted in a 

 given time. 



In order that the escape of moisture from the plant may be 

 as little as possible when the breathing pores must be open to 

 admit air, the great majority of them are placed on the under or 

 shaded side of the leaf. Thus Goodale, quoting from Weiss, 

 gives in a table the number of breathing pores observed per 

 square millimeter of surface on both the under and the upper 

 surfaces of the leaves of forty species of plants, from which it is 

 computed that, on the average in these cases, there are 209 

 breathing pores on the lower side of the leaf for every 51 on the 

 upper side. How numerous and how minute these openings are 

 may be appreciated when it is said that in the forty cases cited 

 there are, on the average, 209,000 stomata on each area the size 

 of the square in Fig. 6, on the under sides of the leaves of these 

 species. Taking a specific case, that of corn, Zea Mays, it is 

 stated that the breathing pores number, on the under side of the 

 leaf, 158, and on the upper side 94, or in all 252 for each square 

 millimeter of leaf, and that the combined area of these openings 

 is .2124 of a square millimeter, so that 21 per cent of the leaf 

 surface of corn is made up of doorways through which air may 

 reach the interior of the plant, and out of which moisture must 

 escape whenever they are open. 



It is not strange, therefore, that large amounts of mois- 

 ture do escape from plants while they are growing, nor that there 

 has been provided a means of checking this loss as far as pos- 

 sible. 



The opening and closing of the guard cells is brought about 

 by changes in the quantity of material which they contain, caus- 

 ing them to open when the cells become distended and to close 

 when they again become limp, Unlike the other o&la ia thg 



