PLANT MICRORADIOGRAPHY 



cleus and nucleolus are particularl}^ well onto the photographic emulsion or pressed 

 visible with the usual microscope. This on it. Microradiographs have 0.2 /x as a 

 choice was very suitable to show biologists resolution, the same as an ordinary micro- 

 what they can hope of microradiography scope. Shrinkages and artefacts must be 

 down to the scale of individuarl cells. From considered in interpretation of microradio- 

 the description (1956) several results were graphs both in ciualitative and quantitative 

 given on resting phase and mitotic sequences methods, but this is true of any technicjue. 

 of the nucleus. Each of them is perfectly The fixation effect is one of the constant 

 recognizable by comparison w4th observa- cares of a cytologist; the desiccating effect 

 tions by the Feulgen staining technique, must be added to it for a microradiologist. 

 Evidently all chromosomes are seen on the The second example offered by literature 

 same plan in microradiographs so that it is on investigation of plants by the high reso- 

 sufficient to localize dry weight inside nu- lution contact microradiography above de- 

 clear structures; but the ordinary micro- scribed concerns pollen in Tradescantia 

 scopic image remains necessary in order to (1957) (21) (Dahl, Rowley and co-workers), 

 count chromosomes with security. The size Diverse phases of development of mother 

 of the nucleolus is about 10 n and diameters cells of pollen were obtained. Here the soft- 

 of the nucleus a few tens of microns. En- ray region used was 6 to 12 A. Specimens 

 largement of microradiographs in (17) were transferred from the X-ray specimen 

 reaches X2000. This successful method holder to a microscope slide and Feulgen re- 

 permits the investigator to locahze and meas- agent. The reference system was made of 

 ure chemical elements responsible for cell narrow strips of nitrocellulose film. High 

 opaque spots. Also it permits experimenta- absorption in the chromosomes was again 

 tion with the help of solvents or enzymes as observed and changes of opacity were 

 carried out by Brattgard and Hyden (1952) studied in mature pollen. 

 (19) on nerve cells. Recently Dietrich (1959) (22) published 

 Cytological Technique of Microradiography results obtained with the same technique on 

 (MR): The physical characteristics of this pollenic material fixed by ethyl alcohol, 

 contact method are utilization of soft X- embedded with paraffin, cut in 2 to 4 /x 

 rays (500 to 2000 V) and high vacuum, sections, floated on distilled water at 35°, 

 The window is a 1000 to 2000 A thick Al coated w^ith film of nitrocellulose, directly 

 foil. Observable area is 12 mm^. Film is a gathered on high resolution emulsion and 

 high resolution photographic emulsion: dried during 6 hours; after the paraffin was 

 Eastman Kodak Spectroscopic Plate type taken off, sections were exposed to x-rays 

 649 and Kodak Maximum Resolution Plate, generated at 700 to 1000 V (= 10 to 20 A) 

 Quantitative measurements can be made on with a copper target. Vacuum was main- 

 the sample microradiographed. Material is tained around the specimen and into the 

 fixed by Flemming or Allen's solutions or demountable tube. Al foil 1 /x protected film 

 freeze-dried before embedding. As shown by and specimen from light exposure. The 

 Engstrom (20) isolated cells require a simple purpose of this work was observation on 

 smearing technique. However, to be ex- changes of opacity inside pollen during 

 amined in vacuo they must be dried. Tissue formation of mature grains and new mem- 

 blocks also after freezing are dehydrated at branes between tetrads. Comparison with 

 low temperature and low pressure, and im- the colored Feulgen technique was used on 

 pregnated with paraffin in vacuo. These thin microradiographed sections. Mitosis and 

 sections have a thickness varying between cytoplasm particularly were observed before, 

 1 and 5 m- They are deparaffined and floated during and after nuclear division. 



641 



