44 



THE SMALLEST LIVING THINGS 



phyll and the products of chlorophyll action are no different from 

 those of certain other organisms which today are almost univer- 

 sally regarded by zoologists as protozoa. This conservatism is 

 based upon the fact that these chlorophyll-bearing forms of pro- 

 tozoa possess definite motile organs in the form of minute vibrat- 

 ing processes called flagella. Each flagellum grows out as a fila- 

 ment from the substance of a kinetic element* of granular form 



Fig. 19— FLAGELLUM 



STRUCTURE OF 



SCYTOMONAS 



From a photomicrograph by 

 the author 



Magnification, 1000 



embedded in the protoplasm. This filament, called the axial fila- 

 ment, is covered by a sheath when it leaves the body (Fig. 19), 

 and this sheath is a prolongation of the outer membrane (peri- 

 plast) which covers the entire organism. Many of the chloro- 

 phyll-bearing forms which are universally accepted as plants, 

 notably the filamentous algae and the thallophytest generally, 

 at certain stages of their activity, form actively motile cells which 

 are called zoospores. Their motile organs are flagella of exactly 

 the same type as in flagellated protozoa, hence recognized plants 

 have the ability to form motile organs. Furthermore in all of 

 the chlorophyll-bearing protozoa, plant-like resting stages, called 

 the palmella phase, are characteristic. At such stages the flagella 

 are lost by resorption and the organisms become embedded in 

 jelly. The length of time in the palmella phase varies — in some 

 groups of the plant flagellates it is the dominant phase (see Fig. 



See page 72, 



t See page 47. 



