1014 



"sind, (lic beiden seitlieli gchi.Lj.erlen Xebcnzclleii diiirli iliicii Turgor- 

 "(Iriu'k den Spaltenverscldnss lierbeifüliren". 



JosT ^) in a very recent handbook deals with the question in 

 the following way: "Die Óffnungsweite der Spalte hangt übrigens 

 "nicht allein von deni Tnrgordruck der Schliesszellen ab sondern 

 "aiich von dem Gegendrnck der Nachbarzellen ; wird dieser etwa 

 "durch Anstecken der Zeilen aufgehoben, so sieht man sofort 

 "eine starke Spaltenötrnnng in den Schliesszellen eintreten, oline 

 "dass in diesen der Druck gestiegen ware. Umgekehrt kann aber 

 "anch eine Druckzunalime in den Nachbarzellen einen passiven 

 "Verschluss der SpaltöfFnnng herl)eiführen. In wie weit indess die 

 "Einwirknng der Nachbarzellen /// der Natur eine RoUe spielt, dar- 

 "über gehen die Meiniingen der Autoren noch weit auseinander; 

 "ScHWENUENER ') schreibt den Nebenzellen gar keine, Lkitgeb ■') eine 

 "sehi- grosse Wichtigkeit zu, Darwin ^) vermittelt". 



It seems to me that the peculiar irregularity in the regulation of 

 evaporation in Viscuiii album and Rki/)salis CVt-ssi/tha may be simply 

 and naturally explained by assuming that when these plants begin 

 to wither the pore of the stoma first widens under tlie influence of 

 the antagonism of the guard-cells and subsidiary cells and is sub- 

 sequently constricted. 



The stomata of Viscum album are rather large and transverse to 

 the long axis of the stem and leaves. In order to survey the com- 

 plicated structure it is necessary to consider sections which have 

 been cut from the appaiatus in different directions and at different levels. 



A section wliich divides the stoma into two halves in a direction 

 perpendicular to the slit gives a view as represented in Fig. 4. The 

 thickened ventral side of the epidermal-cells is seen, the thick ridges 

 which surround the outer porch ("Vorhof") of the stoma, the sub- 

 sidiary cells which to some extent surround the guard-cells above 

 and below, and the very thick collenchymatous ridge which surrounds 

 the inner cavity of the stoma. When a section of the stoma is made 

 in the same direction, not how^ever through the middle, but close to 

 one of the extremities, the figure is different. 



The lumen of the gnard-cells is then (Fig. 5) seen to continue 

 upwards in the shape of a wedge. The dividing- wall between the 

 subsidiary cell and this wedge-shaped continuatioii of the guard-cell 

 is thickened. In the diagrammatic Fig. 1, the lines a, b, and c 



^) JosT. Pflanzenphysiologie. Dritte Auflage 1913. Seite 58. 



^) ScHWE.NDENER 1881. Monatsberlchte Berliner Akademie. S. 833. 



■^) Leitgeb 1886. Mitlheilungen aus dem botanischeu Institut zu Giaz 1. 



^) Darwi.n 1898. Philos. Transactions (B) lüO. 



