155 



thrives well, but suffers often from honey dew. The Robuiise, especially 

 the so-called ball acacia, retain their foliage well even in great drought, but 

 offer only a little shade and put out their leaves late, usually losing them 

 early in autumn. Therefore, when Robinia is planted, arrangements for 

 watering should be made, in which drain pipes perhaps ^ metre below the 

 pavement are put at the distance from the trunk where the newer roots lie. 

 These pipes can be filled when necessary from hydrants. However, atten- 

 tion should be called to the fact that watering through drain pipes can be 

 used only in the hot summer months, because otherwise there would be an 

 excess of water in the soil with much more disasterous results than those 

 due to a scarcity of water. Finally, we think that a sprinkling of the tree 

 crown at night should be recommended especially where watering may be 

 carried out only through the ground about the tree. 



We must emphatically state that watering by means of water drains 

 can be recommended only for light soils with a permeable subsoil. By 

 constantly w^atering heavy soils with a large water content, the soil will be- 

 come baked and compacted, resulting in a scarcity of oxygen and an excess 

 of carbon dioxid as elsewhere described, which combination will bring about 

 the decomposition of the roots. Mangin's^ studies will be cited here as a 

 single warning example. He worked especially on the meagre growth of 

 trees in city planting' and found the soils choked to such an extent that the 

 carbon dioxid content of the soil air increased from i to 5 and 8 per cent, 

 and even to 24 per cent., while the oxygen content fell to 15, 10, 6 and even 

 o per cent. As a matter of course all the trees with such an environment will 

 die. (Compare "Too deep planting of trees," p. 98.) 



Effect of Drought on Field Products. 



The results of continued scarcity of water, felt most cjuickly in sandy 

 soils during great heat, are determined naturally by the time the dry period 

 begins. If it sets in in May, as in 1904, i. e., when growth is most rapid and 

 the activity w^hich should furnish the material for maturing of fruit is re- 

 duced, the effect is most serious. 



In grain, sowing of summer seed suffers most under our cultural con- 

 ditions, when planted at the usual time. This is easily understood when we 

 consider that winter seeds sown in the autumn can, during the whole 

 autumn and the early spring, fully develop their roots and obtain a sufficient 

 formation of shoots. They thus utilize the undisturbed activity of their 

 lower leaves. In this way the winter seed meets the dry period in a strong 

 and well-prepared condition, while summer seed, even where it sprouts 

 normally, enters upon the hot, dry period at a much younger developmental 

 stage. Accordingly the leaves ripen prematurely, their period of work is 

 therefore more limited and even if the plants develop blossoms and the 

 ovaries, comparatively little organic matter is present for filling out the 



1 Mangin, L., Vegetation und Durchliiftung des Bodens. Annal scienc. agro- 

 nom. 2 ser. 1896; cit. Centralbl. f. Agrikulturchemle, 1898, p. 638. 



