DIV. ii PHYSIOLOGY 233 



under special conditions, viz. when the plant is saturated with water 

 and the air is saturated with water vapour. Early in the morning, 

 after a warm, damp, but rainless night, drops of water may be 

 observed on the tips and margins of the leaves of many of the plants 

 of a meadow or garden. The drops gradually increase in size until 

 they finally fall off and are again replaced by smaller drops. These are 

 not dew-drops, although they are often mistaken for them ; on the 

 contrary, these drops of water exude from the leaves themselves. 

 The drops disappear as the sup becomes higher and the air warmer 

 and relatively drier, but can be induced artificially if a glass bell-jar 

 be placed over the plant, or the evaporation in any way diminished. 

 The excretion of jlrops from the leaves 

 can be brought about by artificially forcing 

 water into cut shoots. 



The drops appear at the tips of the leaves 

 in Grasses, on the leaf -teeth of Alchemilla, 

 and from the blunt projections of the leaves in 

 Tropaeolum (Fig. 242). They come from so- 

 called WATER -STOMATA (p. 113) or through 

 ordinary stomata, or they are secreted by small 

 pits or hairs (sometimes by stinging hairs). 

 All such water- excreting organs are termed 



HYDATHODES. 



The excretion of liquid water is far more 

 common in moist tropical forests than in tem- 

 perate climates. Such exudations of water are 

 particularly apparent on many Aroids, and drops 

 of water may often be seen to fall, within short 

 intervals, from the tips of the large leaves. From Fl(; L >42. -Exudation of drops of water 

 the leaves of Colocasia nymphaefolia the exuded from a leaf of Tropaeolum majus. 

 drops of water are even discharged a short dis- (After NOLL.) 

 tance, and 190 drops may fall in a minute from 



a single leaf, while ^ litre may be secreted in the course of a night. Again, 

 in unicellular plants, especially some Moulds, the copious exudation of water is very 

 evident. The water in this case is pressed directly through the cell walls, and 

 in some cases also, as is the case in water plants, through the easily permeable 

 cuticle. 



Since the excretion of water in the liquid form can occur when 

 the conditions are unfavourable to transpiration, especially in sub- 

 merged water plants, it may in a sense take the place of transpiration 

 in maintaining the current from the water-absorbing organs. Its 

 physiological significance is not, however, the same as transpiration, 

 since the expressed water always contains salts, and sometimes also 

 organic substances in solution. In fact, the quantity of salts in water 

 thus exuded is often so abundant that after evaporation a slight 

 incrustation is formed on the leaves (the lime-scales on the leaves of 

 Saxifrages and the masses of salt in some halophytes, p. 240). In 

 some instances, also, the substances in solution in the water may play 



