The pots with a soil moisture under 20 per cent, of the saturation ca- 

 pacity of the sand suffered so much from the summer heat, that the heads 

 in the upper leaf sheaths stood still, without advancing to the formation of 

 kernels. 



In apparent contradiction to such results stand the observations of 

 practical agriculturalists that in perfectly dry, so-called dust-dry soils, the 

 plants can keep on growing, although nutritive substances are entirely lack- 

 ing in the subsoil (it is sterile). Such cases are explicable as soon as the 

 sterile subsoil contains water and the roots remain in the moisture. Haber- 

 landt^ studied this case experimentally. He let the lower part of the roots 

 of his experimental plants dip into distilled water, while the upper roots re- 

 mained in soil layers, which, as shown by control experiments, were so dry 

 that plants wilted in them. The plants of which the outermost roots dipped 

 into distilled water showed a marked increase in dry substances ; from this it 

 is evident that the roots found in the dry soil must have taken up the mineral 

 substances. This division of labor by the roots explains the growth of our 

 cultivated plants in spite of dry surface soil when their roots reach deep into 

 a sterile, but moist, subsoil. 



According to Hellriegel, these changes in production, so well shown in 

 grain, take place in the same sense in other cultivated plants. 



Discoloration of Woody Plants. 



The typical result of a lack of moisture and abundant illumination is 

 the vigorous development of the mechanical tissues. We need refer only 

 to the conditions found in dry climates. For example, Jonsson^ reports that, 

 among other characteristics of arid plants, the walls of the epidermal cells 

 often become slimy. In Haloxylon, Eurotia, Calhgonum, Halimodendron, 

 layers of slime cork alternate with those of common cork. The slime cork 

 is very capable of swelling and is laid bare after the protective cork splits, 

 so that it can take up water and hold it. Cells containing slime are found 

 also in the assimilatory tissues. In Halimodendron, the secondary bark 

 often becomes thick and spongy, thereby modifying the temperature extremes 

 and easily storing up water. In the peripheral parts, abundant secretions of 

 salts form a protection. These characteristics vary in regions where the 

 water supply is abundant in the soil and in the air. Thus, for example, no 

 slime cork is found in Halimodendron when grown in Copenhagen. 



Swanlund^ reports from new Amsterdam on the extremely thick outer 

 walls of the epidermis, the frequent depression of the stomata, the rolling in 

 of the leaves with the resulting restricted transpiration. We have touched 

 upon this subject earlier in the divisions on differences in latitude and on 

 the defects of sandy soils and at the same time have considered the nature of 



1 Cit. Biedermann's Centralbl. f. Agr. Chem. 1878, p. 314. 



2 Jonsson, B., Zur Kenntnis des anatomischen Baues der Wiistenpflanzen. Lunds 

 Univ.-Arsskrift XXXVIII. Bot. Jahresb. 1902, II, p. 292. 



3 Swanlund, J., Die Veg-etation Neu-Amsterdam's und St. Pauli's in ihren 

 Beziehungen zum Klima. Dissert. Basel 1901. 



