THE MOLECULAR FORCES IN PLANTS. 213 



perhnent gave off to the air 4*6 gr., the free water surface 2'23 

 gr. of water. The area of water exposed was 19'6* sq. cm. The 

 surface of the bean leaves is estimated as follows : Very homo- 

 geneous paper is soaked with a solution of Potassium bichromate, 

 and after drying we determine the weight of a piece of the paper 

 of known area. The two primordial leaves of the bean plant are 

 cut off and placed on sufficiently large pieces of the prepared 

 paper, which are then exposed for some time to direct sunlight. 

 The outline of the leaves is soon clearly recognisable on the paper, 

 since the uncovered parts become very brown in colour. We 

 now carefully cut out the portions of the paper corresponding 

 with the leaves, and determine their weight. Given this, and 

 the weight of a piece of paper of known area, we can at once 

 estimate the area of the leaves. I found the total area of the two 

 primordial leaves of my plant to be 230'8 sq. cm. This area of 

 leaf certainly evaporated not more than 4'6 gr. of water, since I 

 have quite neglected the surface of the leaf-stalks of the stem 

 and of the terminal buds, which, of course, also give off small 

 quantities of water to the air. 19'6 sq. cm. of water surface gave 

 off in twenty- four hours 2*23 gr. of water, which is equivalent to 

 1T3 gr. per 100 sq. cm. 230 4 8 sq. cm. of leaf surface lost 4*6 gr. 

 of water in twenty-four hours ; therefore 100 sq. cm. would lose 

 1-99 gr. 



It has already been proved in 81, that the cuticularised regions 

 of the epidermis of leaves have a certain significance in connection 

 with transpiration, since, as we have seen, they are not completely 

 impervious to aqueous vapour. But still it is certain that the 

 greatest importance in this respect belongs to the stomata. It 

 is possible, that is to say, to demonstrate a decided connection 

 between the amount of transpiration of a plant organ and the num- 

 ber of stomata present. A direct proportionality between the rate 

 of transpiration and the number of stomata present on a parti- 

 cular area of leaf is not found, it is true ; but we need not wonder 

 at this, since the aqueous vapour escaping from the stomata is 

 formed, of course, in the intercellular spaces, and the amount 

 of transpiration depends therefore, not only on the number of 

 stomata, but also on the form, size, and number of the inter- 

 cellular spaces. Garreau l has determined the amount of tran- 

 spiration on the upper and lower sides of numerous leaves, and 

 at the same time the number of stomata present on equal areas 

 of leaf surface. He obtained the following results : 



