204 LECTURE XV. 



and INIosses attach themselves to the free surfaces of rocks, e. g. on high moun- 

 tains. The solid crystalline surface of the stone becomes gradually converted, by 

 the activity of the roots of these plants, into a friable crumbling loose substance, 

 which continually penetrates deeper into the stone, and so presents a substratum 

 in which even the stronger roots of larger plants can then obtain a hold. 

 In a certain sense, the roots thus behave towards the mineral constituents of 

 the soil much as the roots of parasites behave towards the tissues of their host 

 plants; we shall return to this subject later, however. 



If we now throw another glance on the matters treated of in the present 

 lecture, the fact is clearly demonstrated that the plant undertakes the absorp- 

 tion of its nutritive matters by means of its own activity, and is by no means 

 simply passive. In the first place the root-hairs, after they have become closely 

 applied to the particles of soil, have to take up the water clinging fast to 

 the latter, as well as the equally closely attached nutritive substances. The 

 next step is to convey these substances to the assimilating organs, the 

 green leaves. The wonderful properties of the wood cell-walls, which are 

 comparable with nothing else, permit the ascent of the water and the small 

 quantities of salt dissolved in it through the stem up to the leaves, but only 

 in so far as these latter themselves regulate this movement according to their 

 needs : when strong light promotes turgescence in the sensitive guard-cells of 

 the stomata, causing them to open, the aqueous vapour escapes from the inter- 

 cellular spaces of the leaf simultaneously with the oxygen of the decomposed 

 carbon dioxide; and as assimilation thus commences, the ascending water cur- 

 rent is set in motion by this continued open condition of the stomata, and, 

 starting from the apex of the root-hairs, conveys the constituents of the soil 

 which are indispensable in assimilation into the tissues of the leaves containing 

 chlorophyll. 



That the roots are not merely passively concerned in the absorption of water 

 from the soil, but are independently and actively engaged in it in consequence 

 of certain irritabilities, I convinced myself first in the autumn of 1859, on ob- 

 serving^ that the leaves of plants of Tobacco and Gourd rooted in flower-pots 

 drooped when the earth was cooled down to a few degrees above zero, and 

 thus absorbed less water than was requisite to cover the feeble transpiration. 

 The warming of the flower-pot sufficed to increase the absorption of water so 

 far that the drooping leaves again became turgid. This phenomenon can also 

 be brought about in summer, if the pots in which sensitive plants are rooted 

 are cooled by surrounding them with pieces of ice. It is not all plants however 

 which are provided with roots so sensitive to temperature, but only those, as it 

 appears, which come from warm climates. Cabbages, for instance, and other native 

 plants did not droop when I cooled their roots. On the other hand, again, 

 it is certain that in all plants the absorbing activity of the roots is invigorated 

 by an increase of temperature up to 25° — 30° C ; as is apparent, for instance, 



' i have described the drooping of leaves when the roots are in earth^too cold for them in the 

 paper, ' Landwirthschaftliche Versuchsstationen^ 1859. I> P- 238. 



