PHYSIOLOGY 



209 



both physical and physiological causes, transpiration is more vigorous during the 

 day than night. Plants like Impatiens parviflora, which droop on warm days, 

 become fresh again at the first approach of night. 



2. EXUDATION OF WATER. The discharge of water in a liquid 

 state by direct exudation is not of so frequent occurrence as its loss 

 by evaporation in the form of 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. These 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 



FIG. 101. 'Exudation of drops of 

 water from a leaf of Tropaeolum 

 mqjus. 



FIG. 192. Resinous covering of the stun of a desert 

 plant (Sareocaulon). (From SCHIMPER'S I'lunt- 

 Geogra.phy.) 



leaves themselves. They are discharged near the apex of the leaves 

 of the Indian Corn, but in the case of Alchemilla from every leaf- 

 tooth, and of the Nasturtium from the ends of the seven main nerves 

 (Fig. 191). The drops disappear as the sun 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. When the plant becomes overcharged with water 

 through the activity of the roots, it is discharged in drops. These 

 are pressed out of special water-stomata (p. 104) or in other cases 

 through the ordinary stomata, or from clefts in the epidermis. 



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