PLANT MICROK \l>lOGR APHY 



x-ray source. When beams of increasing 

 wavelengths pass successively through 

 the same chemical element, each giving 

 a radiograph in constant exposure time, 

 perhaps the first one will produce only 

 blackness of the film matter being trans- 

 parent to short wavelengths; the follow- 

 ing will have a slight impression (but by 

 virtue of differential opacity of matter 

 a corresponding image appears on the 

 film); the last beam will show only a 

 white image, which means the object is 

 c^uite opaque towards the corresponding 

 wavelength used. What seems to be a 

 progressive phenomenon of absorption 

 is due to the ciuality of the chemical 

 element. Another one of different atomic 

 number would have an analogous be- 

 havior towards other wavelengths with 

 a different threshold of opacity. 



7. The image is formed by adjacent areas 

 of several different simple elements 

 differentially absorbing radiation as 

 explained above. The diversity of phases 

 each with its own opacity produces the 

 contrast in the whole image. From the 

 concept of opacity, therefore of absorp- 

 tion, results this one of contrast. This 

 kind of image exists for alloys but prac- 

 tically never for living matter. 



8. A simple element plays an important 

 role in the constitution of plant tissue; 

 living matter produces images composed 

 by heterogeneous areas. Each of these 

 is made of complex substances mixed 

 with much water. When a simple ele- 

 ment prevails in such a cell area, its 

 presence is indicated by a notable opac- 

 ity or notable transparency; this ele- 

 ment can be identified and estimated 

 in amount, but it represents only a more 

 or less important fraction of the total 

 substance from a given area. The mech- 

 anism of absorption, phenomenon of 

 opacity and radiocontrast occur indi- 

 vidually for each simple element of a 

 complex tissue. The entire image is a 



summation of their individual effects 

 and the smallest area always appears as 

 a very small entire image. In fact final 

 interpretation of biological structures is 

 relative, but a biologist works always 

 with the complexity of manifestations 

 of life. 

 9. Highest enlargement obtained today in 

 contact microradiography is X1750 

 with a striated muscle of mouse, more 

 than X2000 with Allium root tip cells 

 (Engstrom) , which are an excellent per- 

 formance in microradiography. 



10. Limit of resolution of a projection mi- 

 croscope is actually the best one with 

 0.1 m; that of the contact method de- 

 pends on resolution power of light mi- 

 croscope with only 0.2 m- These limits 

 concern usual apparatus and no special 

 experimental arrangements. 



11. Total error of ciuantitative microanal- 

 ysis from microradiographs is 2.5%, 

 ten times less than some years ago. 

 The reference system error is more 

 important. 



12. As pointed out above the two optical 

 methods currently used with x-rays to 

 observe microscopic structures are: 



(1) contact microradiography (CMR) ; 

 specimen and film adhere to each 

 other; x-ray image is further en- 

 larged by photographic process; 



(2) projection microradiography 

 (PMR); specimen and film are 

 distant to each other; x-ray beam 

 produces direct image enlarged. 



Both are employed by authors in plant 

 field; respective advantages and disad- 

 vantages decide choice of apparatus. 

 Moreover a third method evolves from 

 x-ray optical laws: 



(3) reflection microradiography 

 (RMR); x-ray beam from speci- 

 men incident at grazing angles on 

 mirrors is reflected to form an en- 

 larged image (cf. p. 672). This 



645 



