38 THE CYTOLOGY AND LIFE-HISTORY OF BACTERIA 



appears that newly formed cell envelopes develop their flagella by outgrowth 

 over an appreciable period of time, and the progressive immaturity of the 

 flagella towards the pole which exhibits a thin area of cell wall indicates that 

 the wall itself is more recently formed at that end of the cell, and that in effect 

 it grows from that pole. It is, in any case, logical to assume that a rigid 

 yet " dead " structure, such as a cell wall, would form in this manner. It 

 could not grow all over its surface, but ccuild approximate to doing so only 

 by constantly tearing and reforming, of which there is no evidence. In those 

 multicellular bacteria which, as described above, form the cell wall at a number 

 of growing points at the junctions of the cell walls and cross-walls [i.e. at the 

 poles of the individual cells), an appearance of uniform growth is given. 



Further information upon the subject of growing points is derived from 

 the researches of Bergersen (1952) upon the effect of sub-lethal concentrations 

 of chloramphenicol on Bact. coli. The bacteria developed typical basophilic 

 concentrations in the cell membrane at various points, and from these grew 

 irregular side-branches. As these branches enlarged the growing points 

 remained at the tip so long as growth persisted, retaining their basophilic 

 character indicative oi the presence of metabolically active nucleoprotcins. 

 The growing points, like all such metabolically-active areas, are also centres 

 of oxidation-reduction activity, and for this reason have been equated with 

 mitochondria by some authorities. However, they bear nc^ resemblance to 



Fig. 13 

 CELL ENVELOPES 

 The cell envelopes observable by electron microscopy. Owing to the opacity of the 

 bacterial cell, little can be seen in most intact specimens under the electron microscope ; 

 however, in sectioned or accidentally disrupted material some detail can be observed. 



(1) Bacillus cereus, section showing one complete cross-wall and a second at an earh' stage 

 of ingrowth. The inner edge of the incomplete septum is lined with material which probably 

 represents the cytoplasmic septum preceding and secreting tlie new cell wall material, but 

 which may be coagulated by the osmium tetro.xide with which the preparation has been 

 treated. .10,000. 



(2) Cell wall of partially disrupted Spirillum sp., showing striated structure and blepharo- 

 plasts at the bases nf the compound flagella. ■: 20,000. 



(3) Cell envelopes of jiartially disrupted Pseiidomonas sp. ; the cell wall is transparent and 

 apparently simple ; the blepharoplasts are visible at the bases of the flagella ; the emergent 

 protoplast still appears to be covered by a membrane. ■' 10,000. 



(4) Where the structure is sufficiently small to be electron transparent the cross-walls 

 can be demonstrated in the intact organism, as in the stalks oiCaulobactcy, which are extensions 

 of the body wall, x 15,000. 



