36 EVOLUTION OF PLANTS 



of disease germs, nearly all of which are bacteria. The 

 great importance of these minute organisms in the 

 economy of nature is at once evident when we reflect 

 that, without their assistance, the decomposition of dead 

 organic matter would practically cease, and it would 

 remain inert and useless as food for the higher plants. 

 The presence of bacteria in the soil is of the greatest 

 importance, as it is through their agency that the nitro- 

 gen compounds are put in such form that they can be 

 absorbed by the roots of the higher plants. 



While the position of the bacteria is unquestionably 

 very low in the scale, their relation to the higher plants 

 is somewhat problematical. The presence of cilia has 

 suggested a possible connection with the flagellate 

 infusorians. Related to them is a peculiar group of 

 simple, green plants known variously as "Cyanophycese " 

 — Blue-green Algae — or " Schizophyceae " — Fission 

 Algse. Like the bacteria they multiply ordinarily by 

 simple, transverse fission, but may also produce resting- 

 spores. Being provided with chlorophyll, however, 

 they are to some extent independent, but they often 

 occur in such positions as to indicate a partial depend- 

 ence on other plants for food. Some occur regularly 

 within the bodies of higher plants, and are probably 

 parasitic to a limited extent. More commonly they live 

 free upon damp earth, or in stagnant water (Fig. 4, 

 A, B, C). 



Like the bacteria, the cell-structure is very simple, 

 and it is doubtful whether a perfectly organized nucleus 

 is ever present, although a central structure of doubtful 

 nature has been considered by some botanists to be a 

 genuine nucleus. They resemble the bacteria, also, in 



