YOUNG OF HIGHER PLANTS - 105 



germinates as a spore does and its nucleus divides to form 

 the nucleus that fertilizes the egg. A real male cell never 

 germinates or divides so far as we know. The pollen does 

 not unite with the egg; one of the cells descended from 

 the pollen unites with the egg. In a similar way the large 

 cell first formed in the ovule, which has been called the 

 large spore (megaspore), is not itself fertilized. It is not 

 an egg. It divides several times, and one of its descend- 

 ants becomes an egg. 



Finally, while there are many differences between the 

 alternation of generations in the fern and the alternation 

 in the higher plants, we can find conditions in plants lying 

 between (connecting links) that prove to us that the 

 higher seed plants have a clear alternation between 

 sporophyte and gametophyte, and show that this 

 alternation in the seed plants is evolved from the 

 alternation in the lower plants. 



6. Changes from Fern to Flowering Plant. The following 

 are the chief changes that would have to come to the fern 

 to make a flowering plant out of it: 



1. Some of the leaves only would bear spores while the 

 others would give up this work altogether. We find this 

 condition among some ferns. 



2. There would need to be produced two kinds of 

 spores, small and large; the small producing a male 

 gametophyte and the large a female gametophyte. This 

 also is found among some of the allies of the ferns. 



3. The size of the gametophyte must be very much 

 smaller than we find in the common fern. Indeed the 

 gametophyte in the seed plant is little larger than the 

 spore. This is true also of some of the fern plants. 



4. The large spores would not escape from the leaf that 

 produces them, but would germinate and the resulting 

 plants would live parasitically on the tissues of the 

 sporophyte. 



5. Because the seed plants are mostly aerial and the 



