356 ANNUAL EECORD OF SCIENCE AND INDUSTRY. 



willow, for instance, shows very little difference. This is, in 

 Mr. Risler's opinion, one of the reasons why some plants thrive 

 better in shade than others. Humidity or motion of the air 

 exerts no appreciable influence upon evaporation, although it 

 was ascertained that evaporation increased with the approach 

 of a storm, even when heat and light had not increased. The 

 moisture of the soil is of the greatest importance. After rain 

 or watering, the evaporation increases ; with continued dr}^- 

 ness, the plants wilt, but frequently recover with the disap- 

 pearance of the sun, when the absorption of water by the 

 roots becomes greater than the evaporation by the leaves. 

 When a large volume of soil is accessible to the plants, the 

 supply of moisture is larger and more regular. Hence the 

 practical conclusions. A crop requires the more water the 

 more densely it is planted. When older and more vigorous 

 plants stand in the same soil with younger and less developed 

 ones, the former will take up more water than the latter, and 

 these will suffer, if the moisture do not sufiice for both. 

 Also, with the water, the stronger plants take more of the 

 nourishment in solution ; hence the injurious influence of trees 

 and weeds upon young plantations. The water taken from 

 that portion of the ground with which the roots are in imme- 

 diate contact is replaced by the surrounding soil, so as to 

 establish an equilibrium of moisture in accordance with the 

 laws of diffusion. A soil will suffer less from dryness the 

 better it is manured. With an equal surface, the evaporation 

 from the young leaves is less than from those that are fully 

 developed. Mr. Risler, from his careful experiments and ob- 

 servations, has also deduced the absolute amount of evapo- 

 ration for several important crops, and ascertained that, in 

 many cases, the quantity of water evaporated is much greater 

 than that obtained from rain during the growing season, the 

 plants drawing their supply from the moisture previously 

 accumulated in the soil. Hence the value of deep culture, 

 which extends the area accessible to the roots. Forests evap- 

 orate much less water than cultivated land : they consume, 

 in fact, not one half of the fallen rain, and thus serve as a 

 source of supply for the adjoining regions. Their ramified 

 roots prevent a too rapid rush of the water, and the washing 

 of the surface soil, etc. ; but Mr. Risler maintains that it is by 

 no means proved that woods actually increase the rain-fall 



