54 Practical Plant Biology. 



emphasising the extreme minuteness of these organisms you must 

 not lose sight of their grossness when compared with molecular 

 dimensions. Thus it has been calculated that the smallest 

 coccus o'i5/x in diameter contains at least 30,000 protein mole- 

 cules, which are of course vastly larger than molecules of 

 simpler substances. They are in fact about the same size as 

 the colloidal particles spoken of in Lecture II. 



It is hot surprising that in cells so minute as these bacteria 

 little structure is visible. In some the protoplasm is seen to 

 contain vacuoles and even granules may be made out embedded 

 in it. When the cell is plasmolysed the cell-wall may be dis- 

 tinctly seen and it has been possible to show that the substance 

 forming this wall is different from that of other plant cells, in 

 that it is a protein and not a carbohydrate. The contents of 

 the cell isolated from the cell-wall give the reactions for pro- 

 teins ; their physical properties, e.g. semi-permeability, colloidal 

 nature, refringency, transparency, etc., correspond to those of 

 protoplasm, as do also their so-called vital properties, viz. their 

 susceptibility to extremes of temperature and to poisons. Hence 

 it is rational to conclude that they are similar to the proto- 

 plasmic contents of other cells. The granules embedded in 

 this protoplasm may often be shown to give the reactions of 

 fats and of proteins. No nucleus can be demonstrated with 

 certainty, but it is impossible to deny that some of the pro- 

 tein granules are nuclei, especially when they give some of 

 the special reactions of nuclear protoplasm. Naturally the 

 structure which characterises the nuclei of larger cells cannot 

 be observed. The small size alone would preclude its observa- 

 tion even if it were existent, and possibly may preclude its 

 existence. As it is we may regard some of these scattered 

 granules either as separate nuclei or as the fragments of a 

 single disintegrated nucleus, or, perhaps, of one not yet organised. 

 The rapid motions of some of the cells leads us to suspect 

 the presence of cilia. This surmise is supported by the ob- 

 servation of suitably stained cells. But the demonstration of 

 cilia in this case is one of great difficulty ; for not only does 

 their extreme tenuity make them difficult to see, but the bac- 

 terial cell exhibits the peculiar idiosyncrasy of casting off its 

 cilia when exposed to any severe shock. Thus it happens that 

 very often the act of killing detaches the cilia, and so when 

 stained no cilia appear. Refined methods, however, have shown 

 that many of the cocci possess a single cilium. Bacilli may 

 be more or less uniformly covered with cilia, they may possess 



