ECOLOGICAL 159 



insects, chokes their breathing tubes, and then serves for digestion. 

 The margins of the leaves are sensitive to touch and turn shghtly 

 inwards, thus assisting capture. Particles of sand may provoke a 

 slight inturning, but this is soon reversed. The effective inrolHng 

 of the margin follows the entanglement of small insects, which 

 points to the need for a chemical as well as a mechanical stimulus. 

 Like Pinguicula in its mode of insect-capture is the Austrahan 

 BybHs. 



Sundews. — Widely distributed on moors all over the world, 

 except in the Arctic regions and the islands of the Pacific, are the 

 sundews (Drosera), of which about a hundred species are known. 

 On tufts of bog-moss or the like one sees little rosettes of red leaves 

 glistening with drops of secretion, which have earned for the plant 

 such names as sundew, Sonnentau, Rossolis. From the centre of the 

 prostrate rosette there rises a short upright stalk, bearing incon- 

 spicuous whitish flowers in summer, or more conspicuous seed-pods 

 in autumn. In the common Drosera rotundifolia a narrow leaf-stalk 

 expands into a more or less circular blade, the edges and surfaces 

 of which are beset with scores of peculiar somewhat club-like 

 glandular hairs or "tentacles", long and recurved at the margin, 

 very short towards the centre of the blade. On an average there are 

 about two hundred, and the longer ones close down, one after the 

 other, on a struggling insect which has stimulated them. Thus 

 the leaf becomes in an hour like a closed fist. The glandular head 

 of the hair exudes digestive juice and a little formic acid, the latter 

 probably serving to prevent premature putrefaction. For about a 

 week the leaf remains closed, and the absorption of the products 

 of digestion goes on. When this is complete, the tentacles begin to 

 move outwards again, the surplus secretion is absorbed, and the 

 leaf reassumes its original condition — with the chitinous debris of 

 the insects remaining on its surface as tell-tale evidences of its 

 prolonged meal. 



The highly evolved hairs are derivable from the glandular hairs 

 familiar on many plants, such as the London Pride and the Catch- 

 fly; and its hould be recalled that digestive ferments are common in 

 many plants — in situations so diverse as the flowers of the Yellow 

 Bedstraw and Artichoke, the stem of Clematis, the seeds of the vetch 

 and the latex of the Papaw Tree {Carica papaya), the leaves of which 

 are sometimes wrapped round flesh to make it tender. But the 

 tentacles of the sundew have evolved far beyond these beginnings. 

 They are very sensitive to stimulus, they are mobile, and they are 

 absorptive as well as digestive. Up the centre of the hair-stalk there 

 runs a bundle of wood-vessels, which supply the gland with water; 

 and outside these there is a layer of elongated cells lined by a thin 

 film of colourless circulating protoplasm, and filled with a purplish 

 fluid. When the gland of the hair is stimulated with a minute 



