1018 



The (wo large crescen I -shaped cells which might at first sight be 

 taken for the guard-cells, are in reality tlie subsidiary cells. When 

 the stoma is similarly cut parallel to the surface but at a lower 

 level, at about the depth of the central slit, it may be represented 

 as in Fig. 11. The section was not completely parallel to the surface 

 of the stem; on one side (the upper one in the iigui-c) the ordinary 

 epidermal cells are cut, on the other side (the lower one in the figure) 

 the section passes through sub-epidermal cells. 



If the stoma is cut at a still lower level, a view may be 

 obtained as lepresented in Fig. 13. Those parts of the walls of the 

 surrounding parenchymatosis cells which border on the internal air 

 si)ace are (generally though not ahvays) somewhat considerably 

 thickened') and in the channel thus formed the two subsidiary cells 

 are seen, separated by a long and \ery nari-ow slit. Fig. 13 was 

 drawn from a preparation viewed tVom the inner side. 



It appears that in the siomiüao( Jihipsa/i'i Qissi/tha consti'\cüon -dud 

 dilatation can take place in three places: 1. At the level of the central 

 slit l)etween the relatively thin parts of the wall of the two guard- 

 cells, 2. at the level of the outer slit I)etween the thick, strongly 

 cuticularised edges which bound this outer slit on both sides, 3. in 

 the plane beneath the guard-cells between the thin-walled i)arts of 

 the subsidiary cells which bulge out downwards. 



The assumption is obvious that in WdimiUs Cassi/t/ia also, at the 

 first withering of the plant, turgor decreases earlier in the subsidiary 

 cells than in the guard-cells and that first, as in VLiCUin album, 

 dilatation of the central slit and increased amount of transi)iration 

 per unit of time, results from this. 



The characteristic peculiarity to which the observed irregularity 

 in transpiration is due in both plants investigated, lies, I consider, 

 principally in the manner in which each guard-cell is surrounded 

 by an e[)idermal-cell which is developed as a subsidiary cell and 

 separates the guard-cell from the other epidermal cells. It seems to 

 me probable that in other plants also, whenever the storaata are 

 constructed in a similar manner, this same irrigularily in the evaporation 

 will be found. In Lomnthus dichrous ot Brazil this same peculiarity 

 seems to occur, less clearly than in Viscum album and Rldpsalis 

 Cassytha, but nevertheless in a very similar manner as the following 

 figures show. 



I) This point was brought out by Vöchting, Beitrage zur Morphologie und 

 Anatomie der Rhipsalideën Pringsheim's Jahrbücher IX. 1873—74. 



Benecke and Westermaier (1. c.) also mention this peculiarity and state their 

 theory of its probable significance. 



