ABSORPTION OF WATER FROM THE SOIL. 259 



small and the compensation afforded by means of the roots is at a minimum; 

 when this is no longer afforded by the roots, equilibrium has nearly set in between 

 the absorbing forces at the surface of the root and the absorptive power of the soil 

 for water. The carrying out of such determinations cannot from their very nature 

 be quite exact, and thus the following numbers are only intended to afford a definite 

 idea of the matters described. Under certain circumstances a young Tobacco 

 plant began to droop when the soil, a mixture of sand and black beech humus 

 (in a room), still contained r2-3<'/o of its dry weight of water, determined at ioo°C. 

 This soil, dried at ioo°C., was able, however, to retain 460/0 of its weight of 

 water by adhesion. Consequently, of the possible amount of water in this soil, 

 only 46 — 12-3 (i.e. 33-7%) was at the disposal of the Tobacco plant; the 

 i2-3<yo of the imbibed water still present was held so fast, that the roots were 

 no longer able to take it up. 



Another almost similar Tobacco plant, standing near this, drooped during 

 a rainy night, when the loamy soil surrounding its roots still contained 80/0 of water ; 

 100 grammes of this loam, however, were able to retain 52- 1 grammes of water by 

 adhesion or absorption ; accordingly this soil, saturated with water, gives up to the 

 plant only 52-1 — 8 (i. e. 44-1%) of its water. 



Under like conditions a third Tobacco plant drooped when its roots were in 

 coarse-grained quartz sand, which, in 100 parts of its weight, still contained 

 1-5% water. This sand, dried at 100° C, was able, however, to retain 20-80/0 

 water; consequently 20-8 — 1-5 (i.e. 19-30/0) of water altogether were at the disposal 

 of the plant, after saturation of the soil had taken place. 



The more the last remnant of water adheres to the particles of soil, the greater 

 also will be the amount of water contained in the soil at the time when the 

 root is no longer able to extract water from it; the humus soil still contained 

 at this time 12-30/0, the loam 80/0, the sand only 1-50/0. Since the drooping 

 only took place when the soU showed this poverty in water, these examples 

 demonstrate that the Tobacco roots still withdraw at least small quantities of 

 water from the soil up to the time when the soil is air-dry, since the propor- 

 tions of water named correspond approximately to the air-dry condition of such 

 soils ; and these researches show at the same time that plants stUl withdraw 

 water from the soil even when it is no longer possible to squeeze any out of it 

 by pressure. 



From these facts, now, it is to be seen that it is by the intimate contact or attach- 

 ment of the root-hairs with the particles of soil that they succeed in sucking up into 

 the plant the extremely thin layers of water of the latter. This union of root-hairs 

 and particles of soil has, however, still another very important significance. Only 

 by this means are they enabled to take from the soil the nutritive materials 

 necessary in addition to the water. In fact, certain materials indispensable for 

 the plant, such as sulphates of lime and magnesia, as well as extremely small 

 traces of other nutritive salts, are without doubt dissolved in the thin layers of 

 water which surround the particles of soil: this follows from the fact that the 

 water running off from the drain pipes of tillage soil contains these substances. 

 But a number of the most valuable nutritive materials are held so fast in vegetable 

 soil, that it is impossible to wash them out with such quantities of water as are 



