1020 



proportion of its weight (varying from 1 "/„ to 4 °/J the amount of 

 transpiration per nnit of time increases and thpn later, when the 

 loss in weight has increased (varying from 6 to 10 Vo). ^'le trans- 

 piration decreases again. 



We may assume that this increase in the intensity of transpiration 

 when the plant first withers is caused by the dilatation of the 

 openings of the stomata, a dilatation which is however only of 

 comparatively short duration and is later again followed by constriction: 



The dilatation of the stomata is i)robal)ly caused by the antagonism 

 between the guard-cells and the subsidiary cells of the stomata in 

 such a way that turgor in the subsidiary cells begins to decrease 

 sooner than in the guard-cells, this phenomenon causing a stronger 

 curvature of the guard-cells and dilatation of the slit of the stomata. 



The subsidiary cells of the stomata, in Viscwn album as in 

 RJupmlis Cassi/thn, surround the guard-cells in a peculiar manner; 

 probably it is in this fact that the cause must be sought for the 

 irregularity of transpiration, with which this paper is concerned. 



EXPLANATION OF THE FIGURES. 



All drawings, except the diagrammalic figure 1 and figure 9, relating to a prepa- 

 ration treated with concentrated sulphuric acid, were made from unstained prepa- 

 rations of spirit material, cut by hand and treated in the usual way. The drawings 

 were made by means of a camera lucida. 



Fig. 1. Diagrammatic transverse section of a stoma of Viscum album. The lines 

 a, b, c, cl, an e give the various directions and levels at which the stoma 

 was cut, line a. corresponds roughly with Fig. 8, h with Fig. 2, c with 



Fig. 6, d with Fig. 7, e with Fig. 8. 

 Fig. 2. Viscum album. From a section parallel to the surface of the leaf. The 



level is roughly that of line h in Fig. 1. 

 Fig. 3. Viscum album. From a section parallel to the surface of the leaf. The 



level is roughly that of line a in Fig. 1. 

 Fig. 4. Viscum album. From a longitudinal section through the leaf. The stoma 



in transverse section, the guard-cells roughly halved. 

 Fig. 5. Viscum album. From a longitudinal section through the leaf Stoma cut 



transversely, close to the extremity. 

 Fig. 0. Viscum album. From a section parallel to the surface of the leaf. Level 



about that of line c in Fig. 1. 

 Fig 7. Viscum album. From a transverse section through the leaf. The stoma 



is cut parallel to the slit, roughly in the direction of the line d in 



Fig. 1. 

 Fig. 8. Visum album. From a transverse section of the leaf. The stoma is cut 



parallel to the slit, in the direction roughly of line e in Fig. 1. The 



preparation was in such a position that the opening of the stoma 



removed by the razor, would have been found on the upper edge. 



