THE PLANT GROUPS 223 
are set apart for special work. 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 be more highly differentiated than 
the Thallophytes, and the Spermatophytes are regarded as 
the most highly differentiated 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 attempt to 
do this work more effectively. It is plant work that makes 
plant structures significant, and hence in this book no at- 
tempt will be made to separate them. All the work of 
plants may be 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 which 
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 plants have descended from the simpler 
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 theory 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. 
