622 ECOLOGY 



at times when the atmospheric humidity is too great for transpiration. 

 Water exudation thereby prevents the injection of air spaces with water 

 and the consequent impairment of respiration and carbohydrate syn- 

 thesis. There are those who regard the injection of air spaces as an 

 imaginary danger and hydathodes as structures of no evident value; the 

 weight of opinion, however, is for the view first stated. 



The mechanics of exudation. The mechanics of exudation appear relatively 

 simple, water being forced through the influence of turgor pressure along the path 

 of least resistance, namely, through the epithem and water stomata. The phenome- 

 non has much in common with Heeding, which may be denned as the exudation of 

 sap from a wounded surface. As with exudation, the amount of bleeding is deter- 

 mined largely by turgor pressure, and it is especially evident when sap is flowing 

 abundantly in spring. Dew and excreted water are likely to be mistaken for one 

 another, particularly as the same atmospheric conditions, facilitate both, and as 

 both are likely to appear on leaf teeth. 



Various water-excreting organs. In addition to undoubted hydathodes, 

 organs are found which have been supposed to excrete water. Among such are 

 structures somewhat like those above described, but on submersed leaves (as in 

 Proserpinaca and Ranunculus}; while transpiration is excluded in such plants, 

 there may be an excretion of liquid water, if there is a conductive stream, as some- 

 times is supposed (p. 610). Water excretion has been thought to be the role of epi- 

 dermal glands which occur close to the terminal tracheids in grooves near the leaf 

 margins of certain ferns. Many hairs (as in Phaseolus and Lathraea), known as 

 trichome hydathodes, have been thought to excrete water through glandular activity. 

 While the exudation of water by these hairs has been doubted, it is easy to believe 

 in such a role because of their close resemblance to nectar glands. Indeed, in the 

 so-called water calyxes of certain tropical plants there are capitate trichome hyda- 

 thodes that excrete large quantities of water and small amounts of sugar, thus grad- 

 ing into true nectaries. In fungi, water exudation may occur at any point on the 

 plant surface, the entire body frequently being covered with drops of water, especially 

 in caves, mines, and moist chambers. 



The influence of external factors upon hydathode development. Little is known 

 concerning the evolution of hydathodes or of the influence of external factors upon 

 their development. When the hydathodes of Conocephalus are poisoned by cor- 

 rosive sublimate, the air spaces become injected with water, and in a few days hyper- 

 trophied tissues are said to protrude from the leaf and excrete water. While these 

 have been called substitute hydathodes, and regarded as a sort of emergency adap- 

 tation, they are probably nothing more than a case of edema (p. 633), appearing, as 

 usual, where an excess of water is present. 



The secretion of oils, resins, and mucilage. Glandular hairs. 

 Glandular hairs are distributed widely in plants, and their secretions 

 are often odoriferous (as in Pelargonium and in the mints), and usually 

 viscid. Most glandular hairs are multicellular epidermal outgrowths 



