36 Mr W. Gardiner, On the Physiological [Nov. 12, 



Other water-glands vary, from this so-called typical structure in 

 many ways, but chiefly in the amount of epithem tissue composing 

 the gland, and in the relative proportion that the cells of this tissue 

 bear to those of the tracheides. Thus, as Yolkens rightly observes, 

 whereas in Dicotyledons there is usually present a well-developed 

 gland, possessing several layers of epithem tissue interposed' 

 between the tracheides and the epidermis, in Monocotyledons the 

 tracheides frequently end directly beneath the epidermis, or are at 

 most, separated from it by one or two layers of cells, which never 

 so far as could be observed, show that marked distinction from the 

 surrounding tissue, as do the cells of the epithem tissue proper. 



In Dicotyledons also great differences prevail, as for instance 

 between the well-developed gland, such as that of Saxifraga crus- 

 tata, with its very numerous layers of epithem, and its distinct and 

 well-defined external sheath (see Fig. I.), and glands of a simpler 

 structure, e.g. that of Soldanella, where the layers are few and 

 where they are not sharply defined from the surrounding tissue. 



The number of water-stoma with which such glands are pro- 

 vided varies within somewhat wide limits. Thus, to take one 

 genus : Crassula coccinea has from 1 — 3, while Crassula spathulata 

 has from 15 — 20, but they may be absent altogether, as in the 

 glands of Alisma and Sagittaria. As noticed, especially by de 

 Bary 1 , the water-stoma differ markedly in size and appearance 

 from the ordinary air-stoma, and as I showed in my paper " On the 

 development of the water-gland of Saxifraga crustata*", they 

 make their appearance long before the ordinary stomata, and 

 their mode of formation, by simple division from a mother-cell is 

 quite different. 



Under appropriate conditions there exudes from the water- 

 stoma, or from the free surface, of such glands as these, drops, 

 which apparently consist practically of pure water, e.g. Fuchsia, 

 &c. or water containing salts in solution — usually carbonates of 

 Calcium and Magnesium, — as in many of the Saxifragaceae and 

 Crassulaceae. 



In the case of water-glands, as indeed in so many of the ques- 

 tions which concern vegetable physiology, it is to Sachs 3 that we 

 owe by far the most important part of our knowledge as to the 

 physiology of these organs, for he it was who clearly pointed out 

 that the exudation of water in water-glands was dependent upon 

 root-pressure, and was most strikingly exhibited at the period of 

 minimum transpiration. He also found that when the activity of 

 the roots was increased by a moderate rise of temperature, there 

 was also an increase in the amount of the exudation. In other 



1 De Bary, loc. cit. p. 54 et seq. 



3 Gardiner, loc. cit. 



3 Sachs, Text Book of Botany, 1883, pp. 676 and 688. 



