CHAPTER XXII 



THE PROBLEM OF SEX IN PLANTS 



316. Cell-division and Reproduction. — As stated in 

 Chapter XIV, the essence of reproduction is the separation, 

 from the body of the parent, of a cell or larger portion, 

 which becomes the starting point of a new individual. 

 In some of the lowest plants, such as certain species of 

 bacteria, cell-division always results in reproduction; that 

 is, the two halves of the divided, one-celled body always 

 separate at the close of cell-division, thus giving rise to 

 two new individuals. A little higher in the scale of life 

 we find such plants as Pleurococcus, where cell-division 

 may result at once in reproduction, but where there is 

 also a marked tendency for the cells to adhere together 

 at the close of division, thus forming a loosely organized, 

 multi-cellular plant body (Fig. 183) . A further advance is 

 illustrated by Spirogyra, where the cells normally do not 

 separate at the close of division, but remain together, end 

 to end, producing a multi- cellular body in the form of a 

 filament (Fig. 251). From this simple condition we have 

 seen transitions to the iiat thallus of the liverworts; the 

 simple, leafy axis of the mosses, the leafless axis of the 

 moss-sporophyte, and the leafy sporophyte of the ferns. 

 Not that these forms are derived from each other; but they 

 illustrate various degrees of complexity from the simplest 

 unicellular plant bod>' to a complex, multi-cellular body. 

 By a comparison of these forms, we see that while, in 



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