330 PRACTICAL BOTANY 



COMPARISON OF THE GREAT DIVISIONS OF 

 THE PLANT KINGDOM 1 



302. Evolution of plants. The leading classes which com- 

 pose the four great divisions of plants have been discussed, 

 and type forms have been described in most of these classes. 

 It has doubtless been made evident throughout Chapters X- 

 XIX that some groups of plants have developed from others. 

 This process is known as the evolution of plants. The oldest 

 plants of the earth were very simple, and from them more 

 complex ones have gradually developed. The simplest plants 

 that are now living probably have changed greatly from the 

 oldest simple plants. It is even probable that some simple 

 plants that are now living have developed from complex forms. 



While we compare one living group with another, we must 

 keep in mind that a higher group of living plants has not 

 necessarily developed from one of the lower living groups, 

 but rather that often in past ages a common ancestry gave rise 

 to both. The lower group has probably changed less than the 

 higher one. While higher groups have evolved from lower 

 ones, it is also possible that certain lower groups of dependent 

 plants have evolved from higher groups. This means that 

 evolution may lead toward greater complexity, as usually hap- 

 pens, or may lead toward greater simplicity in structure. In 

 this summary of the divisions we shall have in mind chiefly 

 two groups of characters, those which relate to nutritive 

 work and those which relate to reproductive work. 



303. Thallophytes. The thallophytes live almost entirely in 

 water or in moist situations, so that usually a fairly constant 

 water supply is present. They are not differentiated into roots, 

 stems, and leaves, and there is little difference between the 

 cells that compose a plant. There are two subdivisions: the 



1 In connection with this general summary the pupil should reread each 

 of the summaries of groups and classifications as they are found at the close 

 of chapters, and review briefly the laboratory work that has been done. 

 Unless this is done with care the final summary will prove very difficult. 



