ABSORPTION OF FLUIDS AND SOLIDS BY SUB-AERIAL ORGANS 159 



while the ash constituents, which the rock may not be able to supply, 

 must be obtained from the dust particles of the air, which are retained 

 by the closely growing tufts and suffice for the needs of the plant when its 

 growth is slow 1 . Direct experiments show that not only the rhizoids 

 but also the leaves of mosses, and the entire thallus of lichens and hepatics, 

 are able to absorb dissolved substances. 



In warmer climes there are numerous epiphytes, which without being 

 compelled to withstand continually recurring desiccation, obtain by means 

 of special arrangements, and owing to the heavy rainfall, sufficient water 

 for all their needs, at the same time receiving the necessary nutrient 

 salts entirely or partially from the air '-', since they have no direct connexion 

 with the soil. 



The white velamen found on the aerial roots of many orchids and 

 aroids is of great functional value in this respect 3 , for, as the change in 

 colour indicates, the dead cells of the velamen absorb the water reaching 

 them in the form of rain or dew, while the inner portions of the root 

 utilize the water thus obtained, absorbing it just as a radicle absorbs 

 water from a wet sponge into which it may have grown. 



In the cases of other epiphytes, such as Oncidium altissimum (Orchid), 

 Anthuriiun Hilgelii (Aroid), Aspleninm serratuui (Fern), the aerial roots 

 form large tangled masses, in which organic debris accumulates often to 

 so great an extent as to form a total bulk of half a cubic metre. In this 

 manner a mass of humus is collected, by means of which the plant is able 

 to obtain water, as well as nutritive material from rain and dew. In 

 Aspleninm nidus-avis and some Bromelias the same end is attained by 

 the leaves becoming arranged like a filter funnel, and in certain of the latter, 

 in which the water is collected mainly in these leaf-funnels, the bases of the 

 leaves are able to absorb water and dissolved substances, and indeed, 

 according to Schimper, may supply the plant with sufficient water for all 

 requirements. In Tillandsia usneoides, whose thin stem and narrow leaves 

 are found hanging freely from branches or even iron beams, the capillary 

 spaces beneath the scale-like hairs suck in dew and rain-water (Schimper), 

 for at these points water can easily be absorbed and transferred to the 

 interior, a power which is absent from the rest of the epidermis. 



Sub-aerial organs are utilized for the absorption of water and dissolved 

 substances to an equally marked extent in certain carnivorous phanerogams, 



1 In fallen snow a large quantity of dust is present, even on mountains, and this is left behind 

 when the snow melts. Reinsch's conclusion (Chem. Centralbl., 1871, p. 520) that higher plants 

 obtain most of their potassium and phosphoric acid from the air is, however, erroneous. 



2 Literature: Schimper, Bot. Centralbl., 1884, Bd. xvil, p. 192, and Bot. Mitth. a. d. Tropen, 

 1888, Heft 2; Goebel, Prlanzenbiol. Schildemngen, 1889, p. 202; Haberlandt, Bot. Tropenreise, 

 1893, p. 172 ; Went, Ann. clu Jard. bot. de Buitenzorg, 1894, T. XII, p. i. 



3 For the structure of these cf. Leitgeb, Denkschr. d. Wiener Akad., 1864, Bd. x.w, p. 179; 

 Meinecke, Flora, 1894, p. 133. 



