THE ABSORPTION OF WATER 



33 



characteristic of being capable of swelling very slightly in water, and hence 

 of permitting very little water to pass through it. The cuticle of the root, 

 on the other hand, is easily permeable to water and swells up into a gelatinous 

 mass (SCHWARZ, 1883). Even the almost waterproof and strongly cutinized 

 cell-walls of the leaf of Sedum fabaria do not appear to be quite impermeable 

 to water, since WIESNER (1882) has observed experimentally that the leaves 

 of this plant, if immersed in water, increase in weight. By means of an old 

 experiment (HALES, 1748, p. 78), easily repeated without special appliances, 

 direct absorption of water by the leaf may be demonstrated. If part of an 

 amputated leafy twig be immersed in water, leaving the cut end and some of 

 the leaves exposed to air, the latter go on transpiring but remain turgid all day 

 long despite the fact that water is being given off from the exposed regions so 

 demonstrating that the submerged leaves are able to absorb as much water as is 

 given off by the exposed leaves. The success of the experiment in any given plant 

 depends entirely on the relative numbers of the absorbing and transpiring leaves. 

 WIESNER (1882) dipped only the apices (bearing some young leaves) of ampu- 

 tated twigs of the vine in water, leaving most of the older leaves to transpire into 

 the air. The result was surprising. The exposed leaves remained turgescent, 

 whilst the apical leaves, although submerged, wilted. In this instance the 

 amount of water absorbed by the apical leaves was insufficient to cover the 

 loss sustained by transpiration. The older leaves withdrew water from the cells 

 of the apex of the branch, and caused them to wilt, even though in water all the 

 time. From what has been said it would appear that aerial parts of plants 

 are also capable of absorbing water, and it would not be difficult to bring 

 forward evidence from the literature on the subject to show that not only leaves 

 and young stems, but also bud-scales and older branches, whose cuticle has been 

 replaced by the still more impermeable cork, can absorb water (KNY, 1895). In 

 our ordinary terrestrial plants, however, during the rainy period, the amount of 

 water absorbed is quite insufficient to cover the loss due to transpiration, and 

 hence the absorption of water through the shoot may, for all practical pur- 

 poses, be ignored. In tropical regions, with a much greater precipitation of 

 moisture, with frequent downpours of rain and greater general dampness of 

 the atmosphere, innumerable plants exist which do not come into contact with 

 the soil at all, and hence can obtain their water supplies from the air only. 

 There are, for instance, the epiphytes, living on the tops of trees, whose bio- 

 logical peculiarities have been described for us in a most attractive manner 

 by SCHIMPER (1888) and GOEBEL (1889). While making a general reference 

 to the works of these authors, we must confine ourselves here to mentioning 

 only a few examples. In the case of many of these epiphytes, as for example, 

 the Araceae and Orchidaceae, long aerial roots are formed whose function it 

 is to absorb water from the air. These roots differ widely in structure from 

 ordinary subterranean roots. Instead of a single layered epidermis producing 

 roothairs, we find a many-layered cellular envelope, the units of which have 

 lost their protoplasm at an early period of life, and which now -form air chambers, 

 communicating with each other and with the exterior by means of pores. 

 When rain falls on this sheath, the drops sink into it as into a sponge, replacing 

 the air in the otherwise empty cells ; from thence the water readily penetrates 

 to the living cells of the root cortex beyond. 



In other epiphytes the roots are reduced in size and serve merely as holdfasts, 

 whilst absorption of water is carried out by the leaves alone. This is seen best, 

 for example, in many Bromeliaceae, where the leaves a*e often arranged in a 

 rosette, their bases enclosing a funnel-shaped cavity in whj^h rain-water accumu- 

 lates as in a cistern. Hairs of special character, quite distinct from roothairs, 

 absorb the water in the pitcher. [These hairs have been lately studied by ME Y 

 ( I 94 Jahrb. f. wiss. Bot. 40, 157) and STEINBRINCK (1905, Flora, 94, 464).] 



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