J 



292 



to be twice as thick at B as at A, it is evident that A will move 

 twice as far from a central axis, C Ci, as B. But this causes the 

 line on which A and B are situated to change from a straight 

 line to a curved one. If two such tubes join each other it will 



leave an opening between them. 



As regards the force here employed, the opinion of the best 

 authorities, including Pfeffer, is in agreement on the two principal 



points : 



I.— That the force is a very considerable one, often amount- 

 ing to several atmospheres ; and 



2. — That it is due to the presence of materials in the cell- 

 lumen of the guard-cell which have the capacity to absorb mois- 

 ture, and thus take it from the neighboring epidermal cells, while 

 its exit is prevented by the protoplasm lining the cell-wall. In 

 this way the cell becomes very turgid, and this turgescence an- 

 swers to the artificially supplied hydrostatic pressure in the rub- 

 ber tube. In a manner too well known to justify its exposition 

 in this connection, it is found that a power equal to five atmos- 



4 



pheres sometimes exists in the cell-lumen of plants. 



FiLiBERT Roth. 



Botanical Laboratory of the University of Michigan, July, iS88. 



F 



EXPLANATION OF FIGURES. 



Fig. i-a. — Stoma of Tradescantia discolor. 



Fig. i-b. — Prunits Laiirocerasus, 



Fig. 2. — Scheme to illustrate bending of a tube with unequal thickness of wall. 



Fig- 3- — Scheme of a guard-cell which projects when at rest. The dotted line 



indicates the same when opened ; a — anterior and b — interior side. 

 Fig. 4. — Scheme to show bending of bar of unequal thickness. (l and 3 after 



Schwendener. 



Schweinfurth's Method of Preserving Plants for Herbaria, 



H. Schenck* calls the attention of collectors, especially those 

 traveling in the tropics, to a method of preserving plants for the 

 herbarium recommended by Schvveinfurth, which he found ex- 

 ceedingly convenient and efficient during his travels in Brazil. 



The plants, when collected, are at once put between the sheets 

 of a leather portfolio. On his return from the excursion the 

 collector places the specimens between single sheets of common 



*ln Bot, Centralbl., vol. xxxv., p. 175. 



.1 



