THE EVOLUTION OF PLANT REPRODUCTION 67 



female cone may find itself in close proximity to the megaspore; 

 at this same time the microspore contents have formed a few- 

 celled male prothallus, part of which bursts through the micro- 

 spore wall and forms a finger-like pollen tube. The pollen tube 

 eats its way through the tissues of the female prothallus to the 

 egg cell, forming a passage way for the male cell which takes the 

 place of the motile sperm. This male cell eventually finds the egg 

 nucleus and then fertilization takes place. All of these events 

 in the sexual phase of alternation of generations are hidden from 

 view within the sporangium on a scale of the female cone. 



From this point on the fertilized egg rapidly forms a minute 

 embryo pine tree with rudimentary root, stem and leaves. At a 

 certain period growth stops and the embryo becomes dormant. 

 While these events are taking place, the integument or coat of the 

 sporangium has been developing into a tough hard covering to 

 protect the embryo; all together form the seed. As the scales of the 

 female cone separate, these seeds float to the ground, in some 

 cases being carried short distances by the wind as a result of 

 wing-like expansions developed by the integument. The dor- 

 mancy of the seed is broken when warmth, moisture and other 

 conditions necessary for germination are present. The embryonic 

 pine then resumes growth, bursts through the seed coats, thrusts 

 its roots into the earth, spreads its foliage to the sun, and starts 

 life as a new sporophyte generation (fig. 39). 



Reproduction in a Flowering Seed Plant 



In a typical Gymnosperm such as a pine, the reproductive 

 organ of the sporophyte — the cone — has but one function: the 

 production of the microspores and megaspores which culminate 

 the asexual cycle in alternation of generations. The cones are not 

 particularly adapted for any special means of ensuring the trans- 

 fer of the microspores (pollen) to the megaspores, gravity or 

 wind being relied upon to bring this about. 



In the Angiosperms (see p. 101) the reproductive organ of the 

 sporophyte is the flower. In many cases this still has the single 

 function of producing the reproductive cells, the plant depending 

 upon the wind to carry the pollen in a hit-or-miss fashion from 

 one plant to another. An example of such a wind-pollinated 



