270 HOUSE, GARDEN, AND FIELD 



is particularly dangerous to green tissues which are laden 

 with water. During a warm day when rain has saturated 

 the earth, absorption of water goes on freely. Even after 

 sundown the ground may still be warm enough to favour 

 rapid absorption by the roots, but the air cools fast, and 

 a temperature low enough to be dangerous to the softer 

 tissues may obtain only a few inches above the warm soil. 

 Under such circumstances grasses and other herbs pass 

 out the water, which has become superfluous and even 

 dangerous, in the form of big drops. Then people generally 

 say that there has been a heavy dew, though it may be 

 that the sky was overcast and that no dew whatever fell. 

 Exuded water may be distinguished from real dew by 

 attending to two points of difference. Dew never forms 

 except under a clear sky ; exudation takes place whenever 

 plants gorged with water are exposed to cold air, whether 

 the sky is clear or cloudy. Secondly, dew forms as minute, 

 close-set drops, which on a surface not easily wetted may 

 afterwards run together to form big drops ; the drops 

 exuded from the water-pores of leaves, on the other hand, 

 are big and solitary from the first. 



The exudation of drops from grass-leaves can be brought 

 about at pleasure. Cut a sod, damp it, lay it on a glass 

 plate, and cover it with a bell-jar. In a day or so the 

 grass, kept at the temperature of an ordinary room, will 

 exude abundantly from the leaf-tips. 



XLVIII. THE WATER-SPIDER. 



A very ingenious predatory animal, which makes use 

 of the properties of the surface-film of water to construct 

 for itself a home beneath the surface, is the water- spider 

 (Argyroneta), of which Prof. Plateau has given a full and 

 interesting account. 1 Like all spiders, this is an air- 

 breathing animal. It dives below the surface, and spends 



1 Bull. Acad. Roy. de Belgique, 1867. 



