2] THE VARIOUS GROUPS OF PLANTS 27 



as frequently as every twenty minutes— but only for a time, the 

 chief limitation being their food supply. They may also form 

 spores which in some cases are highly resistant. Recently, convinc- 

 ing evidence of a sexual process has been obtained in some Bacteria, 

 but it is not known how frequent such a process may be in nature. 



In their modes of nutrition Bacteria vary greatly : for although 

 commonly they are either (i) saprophytic, deriving their energy and 

 materials for life and growth from dead and usually decaying organic 

 matter, or (2) parasitic, depending similarly on living organisms; 

 there are also (3) many forms which can build up their bodies and 

 live from carbon dioxide obtained from the air or water and energy 

 liberated in the oxidation of inorganic compounds or even elements. 

 Such organisms are said to be chemosynthetic, and examples of the 

 substances oxidized by them are sulphur, hydrogen sulphide, nitrites, 

 ammonia, hydrogen, and, apparently, iron and manganese com- 

 pounds. The members of one interesting group of sulphur-oxidizing 

 Bacteria, known as the Purple Bacteria, contain pigments enabling 

 them to absorb radiant energy from light, and they appear to practise 

 some kind of photosynthetic process which may be a prototype of that 

 occurring in ' normal ' green plants. Other types that seem properly 

 referable to the Bacteria are green, through the inclusion of chloro- 

 phyll of a kind. But it seems improbable that the earliest living 

 organisms possessed real chlorophyll or obtained their energy 

 through such an elaborate series of reactions as are involved in 

 photosynthesis {cf. p. 32). Rather is it considered likely that some 

 of these peculiar Bacteria indicate means by which elementary 

 organisms obtained their energy and other requisites of life before 

 either chlorophyll or any form of photosynthesis was evolved. 

 Consequently it seems most reasonable to start our sequence with 

 this group. 



Certain Bacteria are of major importance in causing diseases — 

 particularly of animals, and including some of those most deadly 

 to Man — while various other Bacteria cause the decay and breakdown 

 of dead matter, or make available food-substances for higher plants, 

 or produce chemical ions of many kinds. With their infinitesimal 

 size and often resistant spores, they are among the most widespread 

 of living organisms, being carried by air or water currents or in the 

 bodies of animals practically everywhere in the world and its sur- 

 rounding atmosphere. Nevertheless, as they are so minute, they 

 play only a very minor direct role as components of most types of 

 vegetation. Exceptions are afforded by some aquatic muds, in which 



