658 



show how great the differences can be. He found an average temperature of 

 22.26 degrees C. at 9 A. M. on ten days in August, in soil on which the sun. 

 shone, but, under a cherry tree, a temperature of 19.06 degrees C. In 1884, 

 Wollny^ had already measured the influence of soil shading due to weeds 

 in a potato field and found, at a depth of 10 cm. in the soil, that the temper- 

 ature averaged 2.6 degrees C. less than on a field cleared of weeds. 



Besides the temperature, the amount of water in the soil is of impor- 

 tance. Gain's measurements- show how much the soil moisture influences 

 the size of the leaf. He reckoned the length of the organs set in a dry 

 habitat at 100, the dimensions on damp soil for barley were 240; for poppies 

 550; for potatoes 150. 



If the plants continue to have too little water, their maturing is natu- 

 rally delayed ; their productivity is also considerably reduced. In this 

 connection Bimer's experiments^ should be mentioned. He found that the 

 ripening of potato plants was delayed 8 days in a soil with a 40 to 30 per 

 cent, saturation capacity; 18 days in a 30 to 10 per cent, saturation capacity 

 in contrast to plants with an abundant soil moisture (80 per cent, saturation 

 capacity). With the same high moisture content of the soil, Wollny har- 

 vested 80 g. of tubers from pot plants, while he obtained only 39 g. with 

 half the water content of the soil and only 19.5 g. with 20 per cent, satura- 

 tion capacity. 



In growing herbaceous plants with shallow spreading roots, the yield is 

 markedly decreased by the deeper lying tree roots. In v. Oven's investiga- 

 tions, the water content under a cherry tree amounted to 20.24 per cent., in 

 the unshaded vicinity, however, it amounted to 21.78 per cent. According 

 to Wollny, 2.86 per cent, more water was withdrawn from a potato field by 

 the weeds than by the potatoes alone. 



V. Oven describes the influence of shade on the plant itself, according 

 to his own observations and those of other scientists. The stem members 

 become longer; the leaves more slender and the ripening is retarded. The 

 epidermis, the sheath of the vascular bundles, the walls of the ring ducts 

 and medullary parenchyma are not so thick and the lignification is less. 



The cause of the lengthened period of growth of plants in the shade 

 must be looked for in the lesser intensity of metabolism, which manifests 

 itself in the weaker respiration, since, according to our experiments, the 

 amount of assimilatory activity, under otherwise equal conditions, deter- 

 mines the degree of transpiration, and this also explains the essentially lesser 

 evaporation, and, on this account, the higher water content in shaded plants. 



Of the numerous experiments, which determine a reduction of the 

 harvest due to shade and which v. Oven cites, in addition to his own, one 

 by Wieske on a wheat field is of interest. The plants, which were shaded 

 for the greater part of the day by fruit trees, gave a grain yield decreased 



1 Wollny, Forschungen auf dem Gebiete der Agrikulturphysik, Vol. VII, p. 349. 



2 Bot. Centralbl., Beihefte. Vol. IV, p. 418. 



3 Bimer in Biedermann's Centralbl. 1881, p. 154. 



