X-KAY MICROSCOPY 



localization, either crystals (in rows sur- 

 rounding vessels, superficial deposits or 

 limiting physiologically different tissues) or 

 absorbing zones and poles affecting flow 

 especially of saline solutions. Salts found in 

 higher plants are chiefly calcium ones (car- 

 bonate, tartrate, oxalate under various crys- 

 tallized forms: styloids, raphids, sea ur- 

 chins) and silicaceous formations. Iodine is a 

 possible component of Algae; its high x-ray 

 absorption distinguishes cells containing it. 

 Often kinetics of disintegration of living 

 matter and secretions present some interest- 

 ing information. A remarkable variety of 

 structures in plant tissue and the rapid pro- 

 gression of absorption coefficient with atomic 

 number of plant chemical elements are quite 

 able to be revealed by x-ray microscopy. 



Interpretation of historadiographs is easier 

 when some characteristics of the tissue are 

 already known and problems otherwise 

 studied. Theory and previous trials have 

 indicated approximate radiotransparencies 

 of some groups of substances (pigments, 

 essences, glucids, proteids, some lipids) and 

 radio-opacities of some other groups (tan- 

 nins, resins, salts, some lipids) . Living matter 

 is mainly composed of complexes; these are 

 either organic only or both mixed mineral 

 and organic substances in various propor- 

 tions. What contains the highest concentra- 

 tion of atoms of higher mass per unit area is 

 the most absorbing and opaque. To know 

 what components exist in this area it is 

 possible to employ X-ray diffraction* in 

 case of crystals, or usual chemical micro- 

 analysis. To determine what simple elements 

 build the components, x-ray spectrometric 

 microanalysis is employed. 



Generally C, H and N present no problem 

 because of high transparency; opacity ap- 

 pears with O in molecules when concentra- 



* X-ray diffraction has been employed prin- 

 cipally on membranes and fibers by Frey-Wyssling 

 in higher plants, Preston on Algae and Kreger on 

 coatings from plants. The textile industry uses 

 this method extensively. 



tion is sufficient. High atomic number ele- 

 ments are characterized by strong absorption 

 distinctly visible in the microradiograph. 

 Frequently crystalline structures assure an 

 important contrast in most plant organs 

 conveniently detected by high voltage (15 

 to 20 kV) x-rays in mass tissue, and are 

 relatively easy to observe; but fineness of 

 cell contrasts is so delicate to obtain at 

 these voltages that only the use of soft 

 x-rays permits detection of such order of 

 details in tissue. Low voltages therefore are 

 indispensable to interpret entirely plant 

 structure tissue as it is now possible. 



NOTES ON TECHNIQUES FOR INEXPERIENCED 

 PLANT RESEARCH INVESTIGATORS 



L Microradiographic contrast increases as 

 generator voltage decreases and wave- 

 length increases. 



2. Limit between hard and soft x-rays is 



o ^ 



considered to be 1 A. Hard x-rays are 



o 



below 1 A; soft x-rays employed in bio- 

 logical microradiography are separated 

 in two parts: soft x-rays properly so- 



o 



called (1 to 10 A) and ultrasoft x-rays 

 (10 to 100 A). 



3. Quality of microradiography depends on 

 two main factors: the resolution power 

 which obtains accuracy, and contrast of 

 image which assures the detail of in- 

 formation. 



4. Time of exposure depends on intensity 

 of the beam (this depends itself on focal 

 spot size) and focus-specimen distance, 

 therefore on the method used. It in- 

 creases with thickness of specimen and 

 decreasing grain-size of the film. 



5. The notion of absorption is fundamental 

 in radiography. Absorption by matter 

 depends on x-ray wavelength, nature of 

 elements traversed and their thickness. 



6. Absorption is related to opacity: when 

 a beam of such wavelength passes 

 through the mass of varying chemical 

 composition, an image is registered on 

 film placed behind it relative to the 



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