INTRODUCTION 9 
for the growth of the cell. The power to manufacture 
these carbon compounds is, so far as is positively known, 
confined to cells which contain chlorophyll. Finally, 
by division the cell gives rise to two new ones, which 
become at once independent individuals, each contain- 
ing a nucleus and chromatophores like the parent cell. 
This brief cycle, feeding, growth, and division, consti- 
tutes the whole life-history of many of these lowly 
organisms. 
As we compare these simple plants with the more 
perfect higher forms, we find a more and more marked 
specialization of parts fitting them for special functions. 
Thus there is very early shown a modification of cer- 
tain cells for purely reproductive purposes. These 
cells are evidently descendants of vegetative ones, and 
in their earliest phases of development are often indis- 
tinguishable from the latter ; finally, however, they be- 
come extremely modified, and can serve for reproduction 
only. In extreme cases this results in the formation 
of sexual cells, when two sorts of cells, male and 
female, are produced, each of which is incapable of 
developing further except as the result of a union of 
the two. 
An analogous differentiation of the vegetative parts 
of the plant is seen as we pass from the lower to the 
higher forms. While in the unicellular plant the same 
cell serves to perform all of the functions, in the higher 
plants special organs are developed for special purposes. 
This, of course, reaches its maximum in the seed- 
bearing plants, or “ flowering plants,” as they are more 
commonly known. Here not only is the plant body 
multicellular, but the cells show great variety of form 
