XII. ELECTRON MICROSCOPY .395 



while this limitation is imposed by the necessity of placing the speci- 

 men in a vacuum, if this were not the case, it would be imposed by 

 the necessity of attaining sufficient contrast. Most wet biological 

 preparations would produce little, if any, contrast when examined 

 in the electron microscope. The second part of this condition, 

 namely, the changing of the structure by electron bt)mbardment, 

 might also be considered as part of condition a, but has been inserted 

 here because of the nature of the changes observed. As stated above, 

 the reason for making specimens sufficiently thin is to prevent the 

 electrons transmitted by it from losing energy, which in that case 

 introduces aberrations in the image. However, absorption of energy 

 from the electron beam results in a rise in the temperature of the 

 specimen and, hence, can result in the changing of the structure by 

 evaporation of the somewhat more volatile materials or bj^ chemical 

 or crystallographic changes. 



Fig. 4. Electron micrograph of portion of underside of a leaf showing single 

 stomata. Specimen was prepared bj- a silica replica technique and 

 because of the very rough leaf surface shows some artifacts as a result of 

 folding of the replica. In spite of this great roughness, all parts of the 

 replica are in equally good focus (950 X). 



Fig. 5a. Electron micrograph of Bacterium mycoides grown on nitrogen-free 

 medium to make cells transparent. Made with standard objective lens 

 operated without a diaphragm (5600 X). 



Fig. 5b. Electron micrograph of same field of view as Figure 5a but made with 

 double objective and with an extremely small diaphragm (5600 X). 

 (Specimen prepared by Dr. G. Knaysi, Cornell University.) 



Fig. 6. Electron micrograph of normal cells of Escherichia coli, strain B, 

 grown on supporting membrane by technique described in text and 

 examined with electron microscope using double objective with a small 

 diaphragm (5000 X). 



Fig. 7. Small colony of Escherichia coli grown as described, examined one-half 

 hour after infection with dilute suspension of To bacteriophage particles. 

 Titer of bacteriophage inoculum was so adjusted that each colony 

 would be infected by one bacteriophage particle. Lysis of a single cell 

 is shown in the micrograph. Several bacteriophage particles are visible 

 in remains of the cell but surrounding cells appear normal (9500 X). 



Fig. 8. Preparation of Escherichia coli similar to that of Figure 6, but ex- 

 amined eighteen to twenty hours after infection with T2 bacteriophage. 

 Only a few nonlysed cells can be seen, but field is covered with very 

 large number of bacteriophage particles and lysed cell debris (7200 X). 

 Fig. 9. Electron micrograi)h of bacteriophage in a Streptococcus laclis prepara- 

 tion shadow cast with gold to give a shadow four times longer than 

 height of the organisms (30,000 X ). (Courtesy Iowa State College.) 



