256 • LECTURE XV. 



provided with root-fibres, not so numerous by far or so densely filling up the soil as 

 in the case of the proper land-plants. It is characteristic of the mode of life of the 

 latter that they only flourish, as a rule, when their roots are distributed in a soil which 

 is relatively dry, and only incidentally saturated with water. Thus, just those plants 

 which exhale the largest quantities of aqueous vapour by means of their assimilating 

 leaves are adapted by their roots to a soil in which relatively little water is contained 

 ordinarily, and especially at the time when most water is made use of. In order to 

 understand this completely, it is to be noted that it is in the months of May, June, 

 July, August, and September that the transpiration and nutrition of our cultivated 

 plants, field plants, and forest trees are particularly active, that is, at a time when the 

 earth is completely saturated with water by the rain only now and again, while weeks 

 and months often pass, during which these plants are necessitated to take up by vigor- 

 ous transpiration large quantities of water from a soil which, as the most superficial 

 inspection shows, contains relatively only small quantities of that liquid. And this 

 apparently unfavourable circumstance is really quite necessary for the well-being of 

 the truly terrestrial plants ; since, as is well known, fields of which the soil is too 

 damp are made in a high degree favourable to vegetation by drainage, and the like 

 is true of gardens and forests. The cultivation of plants in greenhouses and rooms 

 also teaches that land-plants which are rooted in flower-pots very easily perish from 

 rotting of the roots if they are watered too often; and it is one of the most 

 elementary rules of the cultivation of plants in pots, to let the earth in the latter 

 become very dry each time before fresh water is poured on. The roots of land-plants 

 thus properly carry on their functions continuously only when the soil surrounding 

 them is as a rule relatively poor in water ; although a complete saturation of the soil 

 for a short time does not at once act injuriously. The condition normally favourable 

 for the roots of transpiring land-plants is therefore this; that they are distributed 

 in a soil which, in addition to small quantities of water, contains at the same time 

 spaces filled with air, by means of which the respiration of the roots is maintained. 

 Very often, however, during long continued drought, the quantity of water contained 

 in the soil sinks so low that the latter appears almost air-dry ; and it seems scarcely 

 intelligible how the roots are able to extract from it the large quantities of water 

 transpired through the leaves. It thus concerns us to obtain an accurate notion 

 as to how the roots of land-plants accomplish the absorption of such large quantities 

 of water, from a soil relatively poor in water. I made this problem the subject of deep 

 reflections and experiments so long ago as 1859 \ and presented the results in a con- 

 nected form in my Handbook of the Experimental Physiology of Plants, 1865. The 

 figure already employed there to illustrate the behaviour of the roots in the soil, may 

 here again serve for further explanation. For the absorption of water, it is only the 

 younger portions of the individual root-fibres, distant some centimetres from the 

 root-cap, which essentially come into consideration — parts which, not yet covered 

 with periderm, are provided with thousands of root-hairs. It is these root-hairs 

 which bring about the immediate connection of land-plants with the soil which 

 nourishes them. We may therefore confine our considerations to the behaviour of 



' For the behaviour of capillary water in the soil, I refer to my treatise in the paper, 

 ■ Laiuhvirthsiliaftl. Versuchsstationen,' 1S59. Heft. IV, p. i. 



