WATER RELATIONS OF PLANTS 



307 



forest. The method proposed by Lloyd is therefore used more 

 frequently. According to this procedure, a strip of epidermis is 

 quickly removed with forceps and plunged into absolute alcohol. 

 This instantly dehydrates the cell walls and preserves them in 

 the condition they were in at the moment of stripping. Such a 

 fixed preparation may be kept indefinitely and examined under 

 the microscope whenever necessary. 

 A still simpler method was 

 devised by Molisch. It is based 

 on the fact that liquids capable of 

 wetting the cuticle will readily 

 penetrate the open stomata and 

 rapidly fill the intercellular spaces 

 of the leaf. Such an injected leaf 

 becomes quite transparent. In 

 order to determine the degree of 

 opening of stomata, a drop of 

 benzene or absolute alcohol is 

 placed on the leaf by means of a 

 small brush or glass rod. If the 

 stomata are open, the drop is rapidly 

 absorbed, and transparent patches 

 appear on the leaf (Fig. 98). If 

 they are closed, the drop will dry 



rapidly without leaving any trace. Fig. 98.— Transparent band 



As benzene can penetrate into nar- f^^^ed after the infiltration with 



benzol across a part of a leaf that 

 rower OpemngS than alcohol, it may was illuminated and therefore had 



infiltrate a leaf with nearly closed «p^^ stomata {after Molisch). 

 stomata. By applying both liquids alternately, it is possible to 

 determine not only whether the stomata are open or closed, but 

 also the approximate degree of opening. Instead of benzene and 

 alcohol, other liquids, such as petroleum ether or kerosene, may 

 be used. The infiltration method is not exact, but it is simple 

 and well adapted for use under natural conditions. 



For determining the degree of opening of the stomata, Francis 

 Darwin devised a special apparatus called by him the *'porom- 

 eter." It consists of two parts: a bell jar about 1 cm. in 

 diameter, and a T-shaped tube (Fig. 99). The tiny bell jar is 

 glued by its flange to the leaf surface. This leaf chamber is 

 connected by a rubber tube with one of the horizontal arms of the 



