292 Science of Plant Life 



manufacture food for themselves. This permits a great in- 

 crease in the size of the plant body, and makes possible a 

 high degree of specialization in plant organs and tissues. 

 As far as is known, the ferns and their relatives were the 

 first plants to develop a conductive system. We do not 

 know how the conductive system originated, but its coming 

 into existence marked perhaps the most important step in 

 the progress of the plant kingdom. 



Summary. The ferns and their allies constitute the rem- 

 nant of a very ancient and important group of spore plants. 

 In the study of the evolution of seed plants they are of in- 

 terest (i) because they have two entirely separate generations, 

 a small, sexual one which produces gametes, and the large, 

 leafy plant which produces asexual spores ; (2) because they 

 show some of the stages in the transition from plants which 

 reproduce only by spores to plants that produce seeds ; and 

 (3) because they were the first plants to develop a conductive 

 system. The presence of a conductive system enables these 

 plants to raise leaves and stems well above the soil and to 

 expose their photosynthetic tissues to the light far better 

 than any of the preceding groups. The ferns do not have 

 extensive root systems, and consequently are more closely 

 confined to moist habitats than are the seed plants. 



