402 A TEXTBOOK OF BOTANY [Ch. X 



extreme simplicity of cellular structure makes them clearly 

 the simplest of existent independent plants, on which ac- 

 count we must place them at the base of the phylogenetic 

 tree (Fig. 275). This fact, in conjunction with their ability 

 to stand heated and murky waters, implies that they prob- 

 ably represent a survival of forms which were dominant at 

 an early epoch of the earth's history when the temperature 

 was higher and chemical action more vigorous than now. It 

 is possible, indeed, that their blue constituent, phycocyanin, 

 is a survival of a functional precursor of chlorophyll. 



Class 2. Schizomycetes : the Bacteria 



These are best known through the " germs'' or " microbes" 

 associated with deadly diseases, though they also include a 

 multitude of harmless and some economically useful forms, 

 — in all about 1400 species. They are the simplest known 

 organisms, consisting typically each of a single cell, of 

 which the protoplasm is hardly at all differentiated. They 

 are also the smallest known organisms, some spherical 

 kinds being only .0005 millimeter (^ inch) in diameter, 

 which is about the wave length of green light, and hence 

 near the resolving power of the microscope, while still smaller 

 and even ultramicroscopic kinds are known to exist. De- 

 spite their apparent structural simplicity and their minute- 

 ness, however (possibly, indeed, in consequence of those 

 features), they possess diverse and remarkable physiological 

 powers. 



The plant body consists typically of a single cell, which 

 may have any one of a number of shapes, as illustrated in 

 Figure 278. These shapes, indeed, form the basis of the no- 

 menclature of the Bacteria, the spherical kinds being called 

 coccus forms, the rod-like, bacillus forms, and the spiral, 

 spirillum forms, while there are others less common. Many 

 possess fine cilia, or else terminal flagella, the movements 

 of which effect a limited locomotion. No nuclei are visible, 

 though chromatin is present in scattered granules, nor 



