KLK( ; i l« )N M l( :k( >SC<)PY 



from l.ctwvfii I he cellulose tape and the 

 "Formvar," but does not affect the latter, 

 wiiich is insoluble in acetone, and keeps the 

 carbon fihn fiat and intact in the solvent. 

 Specinu'ii ^lids are inserted into the space 

 made by the removal of the "Bedacryl" 

 (Fig. Id). Athene, New 200, 3.05 mm grids 

 have been used in this work. The "Formvar" 



Carbon Rodi 



I Carbon Carbon _^,,„, 



Leaf Layer l-oy«r ^'^"l* 



(0) (b) 



(«) 



Support Grid Replica on Grid Replica on Grid 



(<") («) M 



Fig. 1. Stages in the carbon replica technique. 



A'Jf'*^ *.ei>l 



Fig. 2. Adaxial leaf surface of Chrj^santhemum 

 segetum. 



and the carbon film is then lifted on the grid 

 from the acetone bath and dried. The "Form- 

 var" is finally washed away in a chlorofoi'm 

 bath (Fig. le). A final .stage (Fig. If), which 

 is rarely necessary, is to clean the film in a 

 bath of chromic acid. This is only necessary 

 where field material is examined and dirt 

 picked up from the leaf might contaminate 

 the microscope. Finally the replica is shad- 

 dowed with gold/palladium through the grid 

 bars at a fairly high angle, i.e., 1:1 or 2:1. 

 Although originally intended as a high- 

 resolution shadow-casting technique, the 

 method of Bradley (1959) for the simultane- 

 ous evaporation of carbon and platinum has 

 been used to make self-shadowed carbon 

 replicas. The carbon/platinum film is de- 

 posited from 45° or 33° and the later stages 

 of the technique are as described above. Very 

 successful results with this adapted tech- 

 nique have been achieved. The compound 

 replicas are tougher, the shadows more 

 clearly defined and the tedium of two stages 

 of making a replica and shadowing are 

 avoided. The fact that one can, apparently 

 at the same time, shadow and form a com- 

 plete replica of a specimen would appear to 

 be a contradiction. The explanation is that 

 evaporating carbon, but not platinum, does 

 not travel in a straight line. The carbon 

 replica is in a sense a negati^'e of the 

 original leaf surface and the design of the 

 electron microscope reverses original densi- 

 ties in the image. The practice has therefore 

 been adopted of using an extra negative stage 

 in printing from the original electron micro- 

 scope plates. This is contrary to certain 

 conventions in electron microscopy, but it 

 results in dark shadows and light protrusions 

 from the plant surfaces — ^a result which is 

 aesthetically more satisfying and easier to 

 interpret. 



The contact angle of a drop of liquid on a 

 surface is a measure of the wettability of 

 that surface. The contact angle for water of 

 a smooth surface of wax is about 100°. Many 

 plant surfaces have contact angles for water 



178 



