ABSORPTION OF LIQUIDS, ETC. BY LEAVES. 255 



however, small quantities of water can also penetrate into the tissue, according to 

 circumstances, and even any particles of salt contained in the water may enter. 

 This must be concluded with certainty from the fact that leaves permeated by 

 lithium nitrate give up the lithium to the surrounding water. The same follows 

 also from the fact that drops of water which are allowed to cling to the surfaces of 

 leaves show an alkaline reaction after some time, because small quantities of alkaline 

 salts have diffused out from the tissue ^ ; and that substances of the most various kind 

 penetrate from outside into the leaves, is shown by the extraordinary sensitiveness of 

 the latter to vapours of ammonia, chlorine, nitric and sulphuric acids, for example, which 

 injure fields and gardens in the proximity of factories to a large extent. But all this 

 does not prove that any considerable quantities of water, and salts dissolved in it, are 

 conveyed by means of the leaves to the land-plants, and that the activity of the roots 

 and of transpiration is supplemented by this means. When leaves and shoot-axes 

 droop after a hot day, and again become fresh as the sun goes down, this happens 

 in no case by the absorption of aqueous vapour from the air which has again become 

 moist; but simply depends upon the fact that with diminution of the transpiration 

 the turgescence of the organs becomes restored by the continued supply of water 

 from below. 



It appears from what has just been said, that the normal supplying of land- 

 plants with water and nutritive matters dissolved in it, is the function of the roots 

 distributed in the earth. The peculiarities and difficulties, to be spoken of afterwards, 

 with which the absorption of these matters from the soil is connected, as well as the 

 very large quantities of water which must be absorbed, evidently constitute one of 

 the causes which bring it about that transpiring land-plants containing chlorophyll, 

 in contrast to the water-plants and parasites, produce such copiously branched root- 

 systems, consisting mostly of hundreds and thousands of individual root-fibres. I have 

 expressed myself more in detail concerning this in a previous lecture. The copiousness 

 of the root-system corresponds simply to the extent in surface and function of the 

 organs of transpiration. Where, as in submerged water-plants, transpiration is 

 completely wanting, only very few roots or none at all exist; and in the former 

 case these serve in the lower plants (Algae), as has been shown previously, only as 

 anchoring organs. The floating plants provided with an evaporating surface (e. g. 

 Stratiotes, Hydrochan's, Lemnd) have, it is true, more or less developed roots, the 

 function of which, however, is but little in demand, because the leaves only transpire 

 feebly in the moist atmosphere; and, on the other hand, the roots surrounded by 

 liquid water can absorb it without hindrance. Bog-plants are also in a similar 

 position with regard to the activity of their roots : the crown of leaves, it is true, 

 transpires considerable quantities of water, but their roots are in a soil completely 

 saturated with water, and are able to absorb it unhindered. They are in fact much 

 more abundantly rooted than the floating and submerged water-plants ; but still are 



' With regard to the alkaline reaction of drops of water placed on living leaves, and the fact, 

 already discovered by Theodore de Saussure, 1804, that alkalies can be washed out of living leaves, 

 more is to be found in my treatise, ' Ueber alkalische und satire Riaktioncn der Säfte lebender 

 Pßanzenzellen^ Bot. Zeitung, 1862, p. 257. 



