52 PLANT-LIFE 



the preponderance of plant characters until ultimately 

 all traces of a seeming animal nature disappeared. The 

 ciliated zoospores of Ulothrix and other Alga? give rise 

 to a plant thallus. With such a fact before us, we should 

 hesitate before declining to believe that a large number 

 of, if not all, green plants have evolved, by easy stages 

 in the course of ages, from a one-cell flagellated ancestry. 

 Such scepticism becomes the more difficult when we 

 come to consider the egg-cells and spermatozoids of 

 higher plants and the elaborate structures resulting 

 from their fusion. The development of an individual 

 plant from a fertilized egg-cell, going on, as one might 

 say, before one's eyes, may be taken as a recapitulation — 

 at least, in outline — -of the stages of development which 

 slowly led up to the evolution of the individual in a 

 distant past. 



In the evolution of distinctly plant forms we may 

 assume that the elaboration of a cellulose cell-wall was 

 an important advance — one that is suggested by such a 

 type as Chlamydomonas (Fig. 18), which may properly 

 be associated with the Volvoceae, a family of the Green 

 Algae. Specimens of this minute one-cell plant occur 

 in stagnant water, such as contained in pools and 

 ditches. They seem to be most plentiful towards the 

 end of spring and in the autumn. On examining a few 

 specimens under a high power of the microscope, we find 

 that the cell-contents are enclosed in a membrane of 

 cellulose — a fact readily proven by a chemical test. 

 We might conclude, at first sight, that the cell was uni- 

 formly green; but close observation discloses the fact 

 that the green colour (chlorophyll) is confined to a cup- 

 shaped chloroplast, containing a mass of clear proto- 



