170 THE LOSS OF WATER FROM PLANTS 



of grass blades and the tips or edges of the leaves of other plants are often 

 erroneously considered to be dew. 



The process of guttation occurs from specialized structures known as 

 hydathodcs (Fig. 41). These structures are also sometimes referred to as 

 water stomates or ivater pores. As the figure indicates a typical hydathode 

 consists of an enlarged stomate-like opening below which is a rather large 



Fig. 40. Guttation from tomato leaves. Photograph, courtesy of Dr. J. H. Gourley. 



chamber, bordered by a mass of thin-walled, loosely arranged cells called the 

 epithem. The xylem elements of a vascular bundle terminate just below the 

 epithem. 



The exudation of water through hydathodes is considered to result from 

 a pressure which develops in the sap of the xylem elements, and not to any 

 locally developed pressure in the hydathode itself. This pressure is gen- 

 erally believed to be identical with the so-called "root pressure" (Chap. 

 XV). It is supposed that the water is forced from the vessels through the 

 intercellular spaces of the epithem layer and out of the plant through the 

 pore of the hydathode. This water is not pure but contains traces of sugars 

 and other solutes. While the volume of water exuded by most temperate 

 region plants in guttation is usually insignificant, some tropical species lose 

 large quantities in this process. A young leaf of Colacasia nymphae folia, a 



