240 ABSORPTION-CELLS ON LEAVES. 



inserted above to the green leaves, are metamorphosed into little saucers. In one 

 species of Teasel, Dipsacus lacvniatus (see fig. 56 1 ), and in the North American 

 Silphiwn perfoliatwm (fig. 56 2 ) the two sheathing portions (vaginae) of every pair of 

 opposite leaves are connate and form comparatively large and deep funnel-shaped 

 basins, from the middle of which rises the next higher internode of the stem. In 

 several Meadow-rues (Thalictrum galioides and T. simplex) the secondary leaflets, 

 which are opposite one another and shut close, almost like the valves of a mussel, are 

 moulded so as to form cavities for the retention of water, and in many Umbellif erse, 

 such as Heracleum and Angelica, the vagina of each individual leaf is ventricose 

 or inflated, thus forming a sac enveloping the segment of the stem which stands 

 above it. 



These basins, saucers, and dishes are always so placed, relatively to their 

 surroundings, that the water derived from rain and dew is directed into them from 

 the surfaces of the leaves, or by the segment of the stem which rises from their 

 centres, and thus it is that the depressions are filled. Whether in all cases much of 

 the water accumulated is absorbed is certainly open to doubt. In the case of the 

 leaves of the Alchemilla (fig. 52 2 ), which exhibit the phenomenon so conspicuously 

 that the plant has received the popular name of Dew-cup; the absorption of water 

 is, at anyrate, very inconsiderable, and here the retention of the dew secures 

 advantages of a different kind to which we shall presently have occasion to return. 

 On the other hand, it is established that in the case of basins belonging to tall 

 herbaceous plants, particularly such as grow on steppes and prairies where often 

 no rain falls for a long interval, the water collected is absorbed by the glandular 

 hairs and thin-walled epidermal cells developed within them. The fact of this 

 absorption may be proved by a very simple experiment. Let a stem of the 

 Silphiwm, represented in fig. 56 2 , be cut off beneath the pair of connate leaves, which 

 form a basin by their union, and let the cut surface 1 e closed with sealing-wax, so 

 that no water can be taken up by the stem from below. If the water accumulated 

 in the basin is now emptied out, the leaves shortly become flaccid and droop; but if 

 the basin is left full of water, the leaves preserve their freshness a long while and 

 do not begin to wither until all the water has evaporated and disappeared from the 

 basin. If oil is poured upon the collection of water in the basin, so that evapora- 

 tion from the latter is impeded, a constant diminution of the water in the basin is 

 observed notwithstanding; this leads to the conclusion that the water in question is 

 really taken up by the absorption-cells at the bottom of the basin and conveyed to 

 the tissue of the leaf. 



The first thing that strikes one on surveying once more all the plants possessing 

 on their aerial organs special contrivances for water-absorption is that a large 

 proportion of them have taken up their abode in swamps and on the banks of 

 rivers and streams, or if not there, at all events in situations where no danger 

 exists of the ground being thoroughly dried up. No doubt this appears to be 

 inconsistent. How are we to explain the fact that Gentianeae, ashes, willows, alpine 

 roses, bog-mosses, &c, are still in need of water from the atmosphere, when they all 



