THE EVOLUTION OF PLANT REPRODUCTION 



59 



assumes the root-stem-leaf habit of the familiar fern sporophyte 

 (fig. 33) while still attached to the prothallus. As soon as the roots 

 begin to function and growth has resulted in a few leaves, the new 

 plant becomes autotrophic and 

 takes care of itself; the prothallus, 

 having fulfilled its important 

 function, withers and dies. 



The alternation of genera- 

 tions in the fern (fig. 34), as in the 

 moss, depends upon the sporo- 

 phyte for dispersal of new plants. 

 Also, as in the moss, the spore 

 always grows into a gametophyte 

 (the prothallus) which is green 

 and independent. The most nota- 

 ble difference is that in the fern, 

 the sporophyte has become green 

 and independent also instead of 

 being a parasite upon the gameto- 

 phyte. This ability of the sporo- 

 phyte to take care of itself by 

 photosynthesis is one of the major 

 innovations which has enabled 

 plants to so thoroughly cover the 

 land. 



The Reproductive Cycle in the Little 

 Club Moss 



Fig. 33. — Young fern sporo- 

 phyte, with embryonic leaf, grow- 

 ing out of heart-shaped gameto- 

 phyte. The dark area on the 

 gametophyte is the mass of rhi- 

 zoids with soil particles adhering 

 to them. 



Reproductive habits of the 

 higher plants are of immeasurable 

 importance, since we are de- 

 pendent upon their reproductive 



habits for the majority of our foods — ^their fruits and seeds. In 

 addition, the esthetic appeal of plants to gardeners and horti- 

 culturists is largely due to the production of flowers; and the 

 flower is merely the highest expression of a reproductive organ in 

 the sporophyte. An understanding of the formation of flowers, 

 fruits and seeds is therefore an important aspect of any elemen- 



