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



but very conspicuous, incrustation over the surface of the epidermal 

 cells. 



With Fuchsia the same results were obtained, but in this case 

 the activity of the cells appears to be unusually great, since they 

 exude some water in the daytime, and when not covered with a 

 bell-jar. This may be seen in normal, healthy, potted plants. 



From these experiments there appears to be little doubt that 

 the exudation of water, whether dependent on root-pressure or the 

 pressure of certain localized cells, is materially influenced by light, 

 as such, and that the action of light is unfavourable to the exuda- 

 tion and also retards the assumption of turgidity of cells in general. 

 But during darkness a very definite pressure is established, which 

 must, I think, be regarded as an important factor in determining 

 the rapid growth which occurs at that period; at which time 

 moreover such pressure reaches its maximum. 



To return to the consideration of water-glands, it must be 

 stated that the exudation of water from them is dependent not 

 only upon root-pressure, but is also influenced by light. 



I have now to deal in greater detail with certain other points 

 in the structure of water-glands. 



There seems but little doubt, that in all the higher plants 

 which are provided with an apparatus for permitting the exudation 

 of water; whether this apparatus consists of definite water-stomata 

 or of thin walled epidermal cells, placed in connection with a 

 vascular bundle, and whether epithem cells are present or absent; 

 the chief function of such an apparatus is to allow of the escape 

 of superfluous water which would otherwise cause injection of the 

 intercellular spaces, and even rupture of the plant tissue. The 

 presence of this excess of water in the tissue, is in the main de- 

 pendent upon suspended transpiration, in consequence of which 

 a definite and increasing root-pressure is soon apparent. The 

 assumption of the turgid condition of the cells is also materially 

 aided by darkness. Thus such structures are rightly to be re- 

 garded as the safety-valves of the plant. With this Moll agrees. 

 Further, as Volkens suggests, it is extremely probable that in all 

 plants whatsoever there exists some means of relieving excessive 

 hydrostatic pressure. It also seems that there is little doubt that 

 such structures are special provisions for the relieving of pressure 

 in young organs, both from Moll's experiments as to young and 

 old leaves, and from my own in the same direction, which cer- 

 tainly point to such a conclusion. This view is most remarkably 

 confirmed by a study of the development of water-glands, since 

 they are shown to be active at an extremely early stage in the 

 history of the leaf. 



With regard to the epithem tissue there is some difficulty. 

 It would appear probable, as Volkens has pointed out, that in glands 



