THE PLANT URODPS 223 



are set apart for speciul woi'k. The increasing complexity 

 is usually spoken of as differentiation — that is, the setting 

 apart of structures for different kinds of work. Hence the 

 Bryophytes are said to he more higlily diiferentiated than 

 the Thallophytes, and the Spermatophytes are regarded as 

 the most higldy diiferentiated group of plants. 



150. Nutrition and reproduction. — However variable plants 

 may be in complexity, they all do the same general kind of 

 work. Increasing complexity simply means an attemjjt to 

 do this work more effectively. It is plant work that makes 

 plant structures significant, and hence in this book no at- 

 tempt will he made to separate them. All the work of 

 plants may he put under two heads, nutrition and repro- 

 duction, the former including all those processes by which 

 a plant maintains itself, the latter those processes by wliich 

 it produces new plants. In the lowest plants, these two 

 great kinds of work, or functions, as they are called, are 

 not set apart in different regions of the body, but usually 

 the first step toward differentiation is to set apart the re- 

 productive function from the nutritive, and to develop 

 special reproductive organs which are entirely distinct from 

 the general nutritive body. 



151. The evolution of plants. — It is generally supposed that 

 the more complex 2:)lants have descended from the simj^ler 

 ones ; that the Bryophytes have been derived from the Thallo- 

 phytes, and so on. All the groups, therefore, are supposed 

 to be related among themselves in some way, and it is one 

 of the great problems of botany to discover these relation- 

 ships. This theory of the relationship of plant groups is 

 known as the tiieonj of descent, or more generally as evo- 

 lution. To understand any higher group one must study 

 the lower ones related to it, and therefore the attempt of 

 this book will be to trace the evolution of the plant king- 

 dom, by beginning with the simplest forms and noting the 

 gradual increase in complexity until the highest forms are 

 reached. 



