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 aixd 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 



