lo June. 191 -.] Wahr Rcq^uironents of Crop!,. 351 



difficulty of carbon assimilation. This up-hill chemical change requires 

 the presence of very much water in the plant leaf, in addition to the 

 relatively small amount which is decomposed there. 



Transpiration of water takes place chiefly through little pores or 

 openings called stomata,* which are most abundant on the under side of 

 the leaf. These pores or stomata open and close automatically, according 

 as the water supply is greater or less ; but sun and wind modify their 

 control by tending to open them. With the pores open, transpiration 

 proceeds apace, but is naturally more rapid with a dry than a humid- 

 atmosphere. It is also very much greater in light than darkness, and 

 plants wilted at night may appear fresh in the morning. 



Looking to the effect of sun, wind, and a dry atmosphere upon evapoia- 

 tion from the leaves of plants, one would expect that the amount of water 

 lost would in large measure be dependent upon the climate. It has been 

 mentioned that much water is essential to carbon assimilation. This takes 

 place in the green leaves, and only by day. As water is continually 

 passing through the leaves on its way out by day, it follow's that the water 

 is only available as an aid to assimilation in the course of its passage. 

 Now, as transpiration from the leaves is more active in a dry sunny climate, 

 it would seem that more w'ater must pass through the plant to maintain a 

 suitable supply for carbon- assimilation in such a climate than in a dull 

 humid one where it lingers longer in the leaf. In other words, a definite 

 amount of water may be necessary fqr carbon-assimilation at its best, in 

 a certain plant, under a certain intensity of light, and at a given instant, 

 but as transpiration is more rapid in a dry climate, a larger amount of 

 water must be absorbed there than in a humid one in order to maintain 

 equally good water conditions in the leaf. 



Returning now to the figures of Table I., it will be seen that an 

 attempt has been made in each case to fix the transpiration ratio as a 

 specific character of the crop. If however as we have endeavoured to 

 .show, the usefulness of water is dependent, in the first degree, upon the 

 length of time the leaf is able to retain it, it becomes obvious that the 

 transpiration ratio is less a factor depending upon the kind of crop than 

 upon the climatic conditions of the country in which the experiments are 

 carried out. Comparison of the results of the different workers bears 

 in a rough way this theory out. The transpiration ration for production 

 seems to be low'est in the country with the most humid climate, and 

 differences due to crop are generally small as compared to differences 

 due to country. 



While the rate of transpiration from plants is largely dependent upon 

 the physical conditions of climate which determine rate of drying, it 

 cannot be supposed to be altogether so. The cells of the stomata respond 

 to sun and wind, but this response bears no relation to their drying effect. 

 Consideration of the vigour or tone of the plant also comes in. Neverthe- 

 less it is certain that the climatic conditions which cause drying have 

 a very great influence upon the transpiration ratio of growing crops, and 

 that by knowing the relative rates of evaporation from a free water surface 

 at two places the transpiration requirements at one place could be 

 approximately calculated for the other. 



* See Wheat and its Cultivation, by A. E. V. Richardson. Jour. Dept. Agric, Vic, 1912, p. 187, 

 «t seq. 



