128 PLANT PHYSIOLOGY 



equal. Thus, as a rule, the walls of superficial cells are so 

 water-proofed, either by chemical change or by infiltration 

 of the cellulose, that water will be pressed out of other cells 

 before it will pass from them. The so-called water-pores 

 such as occur on the garden nasturtium are merely the 

 openings on the edges of the leaves of cavities at the tips of 

 vascular bundles. The thin-walled cells bordering upon the 

 ducts and tracheids of these bundles squeeze out water into 

 them, the water makes its way toward the surface, escapes 

 into the cavity, and finally passes out through the pore. 

 The excretion of water can, however, be observed on many 

 plants not provided with such highly developed filtering 

 organs. On the leaves of grasses grown under glass in the 

 laboratory, and on the filaments or erect fruiting bodies of 

 various fungi similarly cultivated, water will collect in drops 

 whenever the substratum is so moist that checking the 

 evaporation will raise the sap-pressure to the filtering 

 point. This occurs regularly in grass-plats at night. The 

 "dew" there formed is mainly expressed water rather than 

 moisture condensed from the air. 



A considerable number of plants, especially those growing 

 in damp tropical regions, rid themselves of superfluous water 

 by means of living glandular hairs on the surface, usually 

 the under surfaces of leaves. According to Haberland* these 

 glandular hairs, to which he gives the name hydathodes, 

 press out liquid only when living. 



The liquid passing out of water-pores and excreted by 

 hydathodes is usually a very dilute solution, mainly of 

 mineral substances, with little or no sugar or other organic 

 compounds. These organs are therefore quite different as to 

 their products from nectaries. 



Other water-excreting glands are not uncommon. The 

 accumulation of water in the pitchers of Nepenthes, Sarra- 

 cenia, Darlingtonia, the secretions on the hairs of Drosera, 

 and on the leaves of Dionea, are due to the action of water- 

 glands. The cells composing these glands change not only 



* Haberland, G. In Sitzungsberichte d. Akad. d. Wissensch., Math-phys. 

 Klasse, Bd. 103, Abth. 1, Wien, 1894; ibid. Bd. 104, 1895; also Jahrb. 

 f. wiss. Botanik, Bd. 30, 1897. 



