THE LOSS OF WATER BY THE PLAXT 



167 



abnormal opening of the stomata is observed during hot desic- 

 cating winds, which explains their harmful effect. 



The mechanism of stomata may be studied in various ways. 

 The simplest method would be the direct observation under the 

 microscope of stomatal movements in an uninjured leaf. But on 

 account of their small size and the lack of transparency of the 

 leaves, this method may be used only with few plants. More- 

 over, it is inconvenient to work with a microscope in a field or 

 forest. The method proposed by 

 Lloyd is therefore used more fre- 

 quently. According to this pro- 

 cedure, 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 

 preparative may be kept indefi- 

 nitely, and examined under the 

 microscope whenever necessary. 



A still simpler method was de- 

 vised 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 the stomata, a drop of 

 benzol or of 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. 

 72). If they are closed, the drop will dry rapidly without leaving 

 any trace. As benzol has the capacity to penetrate into narrower 

 openings than are penetrated by alcohol, it may infiltrate a leaf 

 with nearly closed stomata. By applying both liquids, alternately, 

 one is able to determine not only whether the stomata are open or 

 closed, but also the approximate degree of opening. Instead of 



Fig. 72.— Transparent band formed 

 after the infiltration with benzol 

 across a part of a leaf that was 

 illuminated and therefore had open 

 stomata (after Molisch). 



