FOEMS OF BACTEEIA 



15 



finer structure being demonstrated. The whole question must be 

 regarded as still an open one. 



Next, with regard to portions of protoplasm specialised for purposes 

 of food-production, the only case that we know of is that of a section 

 of the sulphur-bacteria, the members of which are coloured purple. 

 The colouring matter — called bacterio-purpurin — is stated to perform 

 the same functions for these bacteria as chlorophyll does for green 

 plants, that is, it is the means by virtue of the possession of which 

 these bacteria, unlike all others, are able to absorb carbon dioxide, and 

 convert it into a more complex compound which can be used as food 

 by them. 



There are many other coloured bacteria, but the colouring matter in 

 these has either been excreted and is found outside the cell, or appears 

 inside the cell as a secretion of the protoplasm. It is not a living 

 substance, nor is it a part of protoplasm specialised to perform a definite 

 function. The cilia should also be mentioned here as being specialised 

 portions of protoplasm. These structures will be fully dealt with in 

 later sections. 



2. Kaw Materials, Intermediate Products, and Secretion Products. 

 These must be treated together, because often we cannot tell whether 

 any particular substance found in the cell 

 is a raw material, or has been formed by 

 the breaking down of protoplasm. In 

 most bacteria no indication of the presence 

 of these materials is given by treatment 

 with stains, but in a few some of these 

 have been demonstrated, which are chiefly 

 in the form of reserve material. When the 

 supply of raw material or any particular 

 form of intermediate products is in ex- 

 cess of the demand, the surplus is put 

 aside and is called a reserve material. 

 Thus, in the higher plants, when the 

 supply of sugar is greater than is imme- 

 diately required, the excess is stored in 

 the form of starch, which is changed back 

 into sugar when sugar is again wanted. 

 A number of these reserve materials have been demonstrated in bacteria, 

 although it is not known exactly how they have arisen. Thus in 

 Spirillum giganteum, the cell in healthy cultures is found to be full of 

 fat-globules, and another reserve material called volutin (Fig. 25a). If a 



Pig. 25.— Spirillum giganteum. (o) 

 Unstained ; (6) stained with methy- 

 lene-blue. Volutin — blue with purple 

 centre. Fat unstained. 



