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 flat 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 body to a complex, multi-cellular body. 
By a comparison of these forms, we see that while, in 
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