ABSORPTION-CELLS ON LEAVES. 241 



grow either in damp meadows, peat-bogs, on the borders of never-failing springs, or 

 in ever-moist ravines, where their requirements in respect of nutrient water and 

 imbibitious water can be supplied all around by means of the roots? A glance at 

 the company in which these plants occur may perhaps lead to a solution of the 

 problem. In the damp meadows and along the margins of springs where gentians, 

 the Sweet-willow, and plants of that kind are found, the Butterwort (Pinguecula), 

 which has been described in earlier pages amongst carnivorous plants, is never 

 absent; whilst wherever the pale cushions of the Bog-moss spring, there also the 

 Sun-dew is certain to spread out its tentacles for the capture of prey. 



With reference to community of site the assumption is warranted that all these 

 plants which nourish under identical conditions of life endeavour to acquire the 

 same material by means of their aerial parts. Now, this material cannot well be 

 other than nitrogen, of which they do not find a sufficient store in the substratum. 

 What then is more natural than that those plants, which are not adapted to the 

 capture of animals, should use their aerial organs, when these are moistened with 

 rain or dew, to take up direct nitric acid and ammonia, which are contained — 

 though in small traces only — in the atmospheric deposits, instead of waiting till 

 compounds of such great importance to them penetrate into the ground where they 

 may chance to be detained at spots whence the roots could only obtain them after 

 long delay and by a highly complicated process ? When one considers that plants, 

 growing amid the sand and detritus of steppes, on ledges, and in crevices of steep 

 rocks, or epiphytic on the bark of trees, are also able to acquire little or no 

 nitrogenous food from the substratum by means of their roots, their especial equip- 

 ment with apparatus for the absorption of atmospheric water becomes explicable on 

 the ground of the latter being the medium of solution and transport of nitrogenous 

 compounds. In the case of epiphytes and of plants growing on steppes or rocks, 

 there is the additional consideration that a supply of pure water, supplemental to that 

 which can be withdrawn from the substratum, must be very welcome to them in 

 dry weather, and that at such times it is a great advantage for the atmospheric 

 water to be absorbed directly by the aerial organs instead of reaching them in a 

 roundabout manner through the substratum. 



If this idea is justified, the atmospheric moisture taken up by the aerial organs 

 with the help of the above-described contrivances, would be of value to the plant 

 chiefly in being a carrier of nitrogenous compounds, and in this acceptation would 

 have to be looked upon as water of imbibition. Whether it is also used, at least in 

 part, as food-material can neither be asserted nor controverted. A separate absorp- 

 tion of water which serves only for motive power, and of that which is in addition 

 employed in the construction of organic compounds does not take place in a plant, 

 it is not possible to make any a priori statement concerning the moisture taken 

 up, as to which part it has to play in the plant. Most probably the allotment of 

 functions is not at all uniform, but varies considerably according to conditions of 

 time, place, and requirement. 



On a former occasion it has been mentioned that small animals are not 



Vol. I. 16 



