X-RAY MICROSCOPY 



The development of film is made in the 

 following solution during 1 min.: 



Metol l.ogr 



Hydroquinone . ... 6 

 Sodium sulphite . . . 37 

 Sodium carbonate. 37 

 Potassium bromide 1 

 Water 1 liter 



First trials can be made with fairly grainy 

 film such as Kodalith and then the final 

 microradiographs with fine-grain film such 

 as Lippmann. After exposure the film is 

 dried and mounted with Canada balsam 

 between glass slides. Photographic enlarge- 

 ment is then possible by usual microscopic 

 processes. Small magnifications are less easy 

 to realize. It is frequently required to re- 

 produce at XlO a whole microradiographed 

 section to compare it with a cell fragment 

 included within, itself greatly enlarged sev- 

 eral hundred times. These small enlarge- 

 ments are difficult to obtain in good quality. 

 To achieve this the film to be photographed 

 is placed betw^een an objective with, a di- 

 aphragm (f = 35 mm) and a divergent lens 

 of 3 cm in diameter in the photographic 

 mounting. In the other part usual mag- 

 nifications with Lippmann film reach ap- 

 proximately X300 without difficulty. 



To complete the information given by 

 contact microradiographs it is sometimes 

 necessary to standardize with some macro- 

 or microscopic experimental tests. Reference 

 systems are numerous and depend upon the 

 scale of chosen contrast and range from 

 nitrocellulose foils to impregnated papers 

 illustrated in Fig. 2, PL 1. In this case ash- 

 less paper of known thickness is observed 

 with an increasing saline mass per unit area 

 after evaporation and macroradiography. 

 Fig. 7, PI. 1 shows another system with 

 saline solutions in small and long cylinders 

 of identical diameter whose area is here 

 visible. Comparative opacity measures the 

 relative degrees of absorption. 



After Clark (1955) (11) published excellent 



microradiographs of white oak sections, a 

 botanical example was illustrated by Nixon 

 (1956) (12) with the point projection system 

 constructed by Cosslett and Nixon, using 

 two electron lenses to obtain a high demag- 

 nification of the electron source (5-20 kV, 

 XlOOO). This was a bean section whose 

 accuracy represents conspicuously a valu- 

 able quality of micrographs obtained by 

 this method. Jackson (1956) (13) studied 

 in the Cavendish Laboratory untreated 

 specimens of obeche infected with a staining 

 fungus and mahogany sections by means of 

 projection microscope; then abnormal elm 

 burrs sections were presented by the same 

 process (point projection microradiography; 

 target = copper foil 3 ju ; accelerating voltage 

 10 or 15 kV; normal exposure time of 4 

 min.; target-plate distance = 6 cm). 



By the projection method Ely (1956) (14) 

 also studied crystals of Rosa leaves and their 

 seasonal variations (7 to 15 kV; 200 mA; 

 15 sec. to 5 min. exposure). 



Ong and Le Poole (1958) (15) by means 

 of a technique based on projection micros- 

 copy but borrowing from electron micros- 

 copy the methods of shadowcasting and 

 replicas, have obtained relief effects and in- 

 teresting details concerning certain struc- 

 tures of paper fibers and a section of ashwood 

 50 /x thick, diatoms and the stigma of a leaf 

 with crystals. This method permits enlarge- 

 ments up to X520, with an ultimate possi- 

 bility of XlOOO (anode voltage = 6-12 kV; 

 2-15 min. exposure with Al target, 10 sec. 

 wdth Au white radiation). 



The highest point of progress in micro- 

 radiography of plant specimens is the re- 

 markable work on extremely thin sections 

 presented by Engstrom and co-workers 

 (1950-1957) (16). To develope this method 

 accessory techniques were studied. An 

 example given by Greulich and Engstrom 

 (1956) (17) and then by Engstrom and 

 Lundberg (18) in plant field is Allium root 

 tip sections; the onion is a Monocotyledon 

 which therefore possess great cell size. Nu- 



638 



