Seed-Plants 129 



sporophyte, so that the ripe seed comprises struc- 

 tures belonging to three generations. 



Advantages of the Seed Habit. The advantages 

 of the seed habit are apparent, and it is evident that 

 this has resulted in a type of plant peculiarly adapted 

 to life on land. This is shown by the extraordinary 

 development of seed-bearing plants at the present 

 time. Among the pteridophytes, except in Selagi- 

 nella, which is probably the highest genus of living 

 pteridophytes, the developing gametophyte is ex- 

 posed to the vicissitudes of an uncertain water sup- 

 ply, free water being essential to its development 

 and for fertilization. In the seed-plants the gameto- 

 phyte is largely protected during its development, 

 receiving its water supply indirectly through the 

 tissues of the sporophyte, and water is no longer 

 necessary for the fertilization of the ovum, owing 

 to the formation of the pollen-tube. Moreover, the 

 young sporophyte is very perfectly protected during 

 its early development, and before the seed ripens it 

 reaches a condition where it is ready quickly to as- 

 sume an independent condition. 



The food stored up in the seed during the 

 process of ripening, provides an ample supply 

 of food for the young sporophyte during the 

 early stages of germination. These conditions 

 give the seed-plants a tremendous advantage over 

 most pteridophytes, except under very special 

 conditions, either where the latter have developed 

 extraordinary power of vegetative reproduction, 



