THE PLANT-BODY 13 



and by oxidation, the food elements are decomposed and recombined 

 into the organic compounds needed to build up the cell. These 

 unicellular plants are very often actively motile, a condition which 

 in the higher plant-forms is usually restricted to special cells. These 

 active movements are due to cilia delicate vibratile protoplasmic 

 threads which propel the cell through the water. This free-swim- 

 ming condition is probably more primitive than that in which the 

 cells are stationary, and such motile plants show evident rela- 

 tion to similar unicellular animals. The occurrence of such 

 ciljated cells in many of the lower plants, and the frequent reversion 

 to the free-swimming condition in the reproductive cells of the 

 higher ones, indicate that the earliest plant-forms were probably 

 actively motile, and much like the simpler existing Volvocacese. 



Reproduction 



The simplest form of reproduction in these unicellular plants is 

 by mere fission, or the division of the cell into equal parts, each of 

 which becomes at once a complete organism like the original one. 

 Sometimes, instead of the whole cell dividing, it is simply the cell- 

 contents, which divide into two or more parts, each with its own 

 nucleus and chromatophore. These new cells escape from the old 

 one, most often as ciliated bodies, which may at once come to rest 

 and form a new individual, or two of them may fuse into a single 

 cell. This is the simplest type of sexual reproduction, and is absent 

 in a good many of the lower plant-forms, like the Bacteria and Blue- 

 green Algse, in which reproduction is always strictly non-sexual. 



It is clear, then, that a single green cell can feed, respire, grow, 

 move, and reproduce ; in short, can perform all the vital functions 

 which are essential to the existence of the most highly differen- 

 tiated plant or animal. 



Unicellular Plants 



While the typical unicellular plants possess a definite nucleus and 

 chromatophore, there are still simpler forms, like the Bacteria and 

 Blue-green Algse, in which a definite nucleus cannot certainly be 

 demonstrated, and in which either no chromatophore is present, 

 or it is imperfectly differentiated. Of course where no chlorophyll 

 is present, the organism is dependent to some extent upon organic 

 food. 



More advanced than these, and perhaps to be regarded as the 

 starting point for the development of the higher plant-forms, are 

 the free-swimming organisms related apparently to the flagellate 

 Infusorians, from which they differ in the presence of a chromato- 



