218 SUMMARY OF CURRENT RESEARCHES RELATING TO 



decreases with the degree of turgidity. When there is no tension 

 the ventral wall projects ; and the thin spot then acts as the hinge 

 between the thickenings. 



The result of the movement is (in Helleborus) that the anterior 

 chamber of the stoma remains unchanged, while the posterior chamber 

 is greatly narrowed by the closing ; the ventral walls of the guard- 

 cells turning on their outer lines of attachment, and bending con- 

 siderably. The mechanical nature of this process may be determined 

 by observing the change in form of the cell-cavity. When there is no 

 tension, the transverse section of the cavity represents a scalene 

 triangle, pressure tending to change it to an equilateral form, which 

 causes the movement. That this must be the case the author has 

 proved by an experimental apparatus constructed for the purpose. 



As regards the purpose of each separate part of the stoma, the 

 two thickening-ridges may be compared to a half-open portfolio ; the 

 delicate lamella of cell-wall which unites them to the hinge or back. 

 The uniform strength of the thickening-ridges from one end to the 

 other of the fissure is a contrivance to assist the curvature. When 

 the posterior chamber of the stoma is enlarged by the increased 

 turgidity of the guard-cells, the breadth of the hinge increases, as is 

 essential. Turgidity then causes, firstly, a curvature of the cells, and 

 in the second place an enlargement of the posterior chamber. With 

 increase of age the thickenings become stronger, the opening of the 

 stoma being thus rendered more difficult, and, finally, impossible. In 

 many cases they are ultimately closed by thyllose structures. 



The turgidity of the guard-cells is dependent on the influence of 

 light. The fissure was always open (in Amaryllis formosissima) after 

 the plant had been exposed for from one to two hours to direct sun- 

 light ; while the stomata were always closed when the plant had 

 remained for some time in the dark. Within ordinary variations of 

 temperature heat alone does not cause the fissure to open. 



Callus-plates of Sieve-tubes.* — E. Eussow has successfully em- 

 ployed aniline blue for colouring the callus-jilates of sieve-tubes. An 

 aqueous solution of this pigment is taken up in larger quantities and 

 more firmly held by these plates than by the other parts of the sieve- 

 tubes, out of which it can be washed by water. The same effect was 

 not produced by other aniline dyes, as aniline brown. The fine 

 structure was best exhibited by treating it with chloriodide of zinc con- 

 taining an excess of potassium iodide either before or after colouring 

 with the aniline blue. 



Out of a large number of species examined, Eussow found callus- 

 plates in Alsopliila australis, Balantium antarcticum, Osmunda regalis, 

 Equisetmn arvense (but not in Pteris aquilina, Marsilea, or Lycopo' 

 dium) ; and in all families of Gymnosperms, Monocotyledons, and 

 Dicotyledons. 



In Abies Pichta large callus-cushions were found, composed of 

 radially arranged parts with a crystalline appearance, which were 

 evidently doubly refractive. The callus-layers of the sieve-plates of 



* SB. Dorputer Natiuf.-Ges., 1881, April 23. See Bot. Ztg., xxxix. (1881) 

 p. 723. 



