16 PLANT-LIFE 



The existence of a nucleus in the bacterial cell has 

 not yet been satisfactorily demonstrated. The cell 

 is enclosed in a cell-wall, which is generally protein in 

 character, and is thus very similar to the animal cell- 

 wall. Cellulose, as the usual substance of vegetable 

 cell-walls is iiamed, occurs in some Bacteria. The cell- 

 wall is permeable by water, and it is always through the 

 wall that the organism absorbs its nourishment in solu- 

 tion. Bacteria cannot engulf and digest solid particles 

 of food after the manner of animals. There are a few 

 Bacteria which are neither parasites nor saprophytes, 

 as, like the Purple Bacteria already referred to (p. 15), 

 they derive their food from inorganic sources. In this 

 respect they resemble the green plants. With a few ex- 

 ceptions the organisms are colourless : the Sulphur Bac- 

 teria possess a purple pigment, and there is green 

 colouring-matter, regarded by some investigators as 

 chlorophyll, the colouring-matter of ordinary green 

 plants, in Bacterium viride and B. chlorinum. Possibly 

 the green species ought to be classed among the Blue- 

 Green Algae, to which reference will shortly be made. 

 Most Bacteria are not motUe, yet many forms have 

 power of locomotion, due to the possession of proto- 

 plasmic lashes (cilia, or flagella), with which they lash 

 the fluid medium in which they exist, and so secure 

 movement. These lashes are so extremely fine that 

 they do not appear in microscopic examination unless 

 stained. 



In order to form a definite conception of the life- 

 cycle of a Bacterium, we will consider one of the rod-like 

 forms possessing flagella — Bacillus svhtilis (Fig. 7). A 

 plentiful supply can be readily obtained by boiling a 



