1883.] Significance of Water Glands and Nectaries. 37 



words, whatever increased the root-pressure increased the exudation 

 of drops of water. 



More recently there has appeared a paper by Moll 1 , which also 

 deals especially with the physiology of water-exudation by leaves. 

 The experiments of this observer principally consisted in substi- 

 tuting for the normal root-pressure, that of a column of mercury, 

 so that the amount of pressure required to bring about the exuda- 

 tion might both be registered and varied at will. As a whole the 

 results obtained in this way are satisfactory, although it would 

 appear, that this cannot be said of certain of those results, which 

 were the outcome of very great and certainly very abnormal pres- 

 sure, not only causing excessive injection of the tissue, but even in 

 some cases a mechanical exudation of water over the whole surface 

 of the leaf. Great interest is however attached to his observation 

 that it is especially in young leaves that exudation most readily 

 occurs, and that as they increase in age, exudation becomes more 

 difficult or even impossible. Thus, with one or two exceptions, 

 which I shall deal with later on, Moll's results confirm those of 

 Sachs, that the exudation of water by water-glands is dependent 

 upon root-pressure. 



As regard Volkens' 2 conclusions, he finds that although in the 

 greater number of instances, well-developed water-glauds and 

 stoma are present in the leaves of all the higher plants, yet that 

 in certain orders e.g. Resedaceae, Linaceae and Malvaceae no such 

 structures can be detected. In the Papilionaceae he makes the 

 very interesting observation, that the stomata of the upper side of 

 the leaf are larger in size than those of the under surface, and 

 suggests that in this instance the exudation of water takes place 

 by means of these slightly modified stomata. Like myself he 

 believes that a mechanism for permitting of the escape of water is 

 present in some form or other in all plants whatsoever, and in the 

 light of his own, and of Moll's researches, there seems some proba- 

 bility, that in certain cases this function is even performed by 

 ordinary undifferentiated stomata. 



Volkens also draws attention to the difference between the 

 glands of typical Monocotyledons and Dicotyledons. He suggests 

 that water-glands may be regarded as the safety-valves of the 

 plant, and that the epithem tissue acts as a resistance, put in, 

 between the end of the vascular bundle and the free surface, so that 

 for the expulsion of the water, through the water-stoma, a definite 

 pressure should be required, and that the water should not merely 

 run out, as out of a pipe. 



Having thus stated, in a very brief manner, the principal con- 

 clusions with regard to the function of water-glands, I may now 



1 Moll, Bot. Zeit. xxxvm, 1880, p. 25. * Volkens, loc. cit. 



