50 



PLANTS AND MAN 



regard to specialization of fertilization methods, in dispersal of 

 spores during asexual reproduction, in protection and nourish- 

 ment of the embryo within a seed, and in dispersal of the young 

 plants through development of fruits. 



Reproduction in Unicellular Plants 



As might be expected, the simplest form of reproduction is 

 found among the unicellular plants. This is an asexual reproduc- 

 tive process known as fission, closely related to growth; both 



processes being the result of cell division. 

 Protococcus, a microscopic unicellular 

 green plant related to the desmids, lives 

 in colonies on the bark of trees, where 

 it often forms a conspicuous green coat- 

 ing. After the plant has elaborated suffi- 

 cient food materials to enable the cell to 

 attain its maximum dimensions, it un- 

 dergoes cell division (fig. 25). The 

 nucleus first passes through a complex 

 cycle, dividing its chromatin material 

 into equal portions at opposite ends of 

 the cell. Then the cytoplasm around one 

 of these daughter nuclei becomes sepa- 

 rated by a cross wall from that in the 

 other half of the cell surrounding the 

 other nucleus. Thus each half of the orig- 

 inal cell becomes a new cell, entirely 

 surrounded by a cell wall and contain- 

 ing a nucleus. If these cells were to remain attached to each 

 other after cell division, it would be a growth process; and if 

 repeated at intervals would result in a multicellular body. But 

 instead, these two new cells separate from each other and a new 

 generation of two individuals results. This type of reproduction 

 by fission represents the first attempts of life to perpetuate itself, 

 and the entire cell organism takes part in the process. After 

 reproduction by fission, the parent cell loses its identity in the 

 formation of the two new organisms. 



Fig. 25. — Protococcus 

 plants grow and repro- 

 duce by the simple process 

 of cell division. 



