ovule grows in its place inside the ovary in the pistil into 

 a small female gametophyte. The small male gametophyte 

 produces two sperms, one of which unites with the egg 

 that is formed in the female gametophyte. 



In other words, the old view thought of plants as 

 reproducing by the sexual method merely, as in the 

 higher animals. The new view states that seed-plants 

 have an alternation of generations just as the fern has. 

 It states that the seed plant has a generation that produces 

 spores, and these in turn produce another generation that 

 forms gametes. According to the old view the embryo in 

 the acorn is the child of the oak tree. According to the 

 new view it is the grandchild. 



5. What are the Proofs that there is an Alternation 

 of Generations in Seed Plants? It is not possible here 

 to give all the facts that convince the botanist that the 

 modern view is the right one, but there are two or three 

 things that may make it seem reasonable even to a 

 beginner, Everybody agrees that a new generation starts 

 when gametes unite. So when the nucleus from the pollen 

 tube unites with the nucleus of the egg cell, and this 

 fertilized cell begins to grow, we are sure we have a new 

 plant started. We can trace the growth of this plant at all 

 stages into the seed, and through germination, and on to 

 maturity. Furthermore, when these cells unite we have 

 the 2X chromosomes that we found in the sporophyte of 

 the moss and the fern; and this 2X condition of the nucleus 

 continues all through the life of the flowering plant. 



In the second place, when the pollen is formed, it is 

 formed very much like the spores in the fern. The 

 chromosomes are reduced to the X stage as they are in 

 the spore-formation of ferns. The pollen furthermore 

 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 are 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 

 female cell is formed deep in the tissues of the sporophyte, 

 a delicate motile sperm cell could not reach the egg. 



Hence the small spore in germinating produces a tube 

 that penetrates the tissues and allows the sperms to pass 

 down through it. Sperms of ferns and mosses have cilia, 

 and are active in motion. The sperms of seed plants 

 evidently have no need for activity of motion, yet we find 

 that the sperms of some of the lower seed plants have 

 cilia, which strongly suggests that their ancestors had 

 need for ciliated and actively moving sperms. 



CHAPTER TWENTY. 

 FURTHER RELATIONS OF PARENTS AND OFF- 

 SPRING. 



1. Review. In Chapter Eight we studied these relations 

 in some of the lower plants and animals. We found that 

 reproduction is always a sacrifice on the part of the 

 parent; that this sacrifice is necessary to keep the species 

 going; that the parent is completely destroyed in the 

 lowest forms of reproduction, as fission; and that the 

 parent can economize by giving a smaller part of itself 

 to each offspring. Thus we saw a- reduction in the amount 

 of substance which went into each offspring. 



2. What Parents Must Do. Just the same problems 

 confront the parent in the higher forms as were discovered 

 in the lower. Two parents must, on the average, bring to 

 maturity two offspring during their own life time in order 

 to keep the race where it now is. The problem of parents 

 is to hit upon the best way to do this. It is for the good 

 of the race that the parent shall not be used up in this 

 sacrifice until enough offspring have been produced to 

 allow two to survive the disasters that confront them. 

 On the other hand, the parent must sacrifice enough 

 matter and energy in reproduction to insure the species. 



3. Condition in Higher Organisms. In the higher plant 

 and animals we have found two methods of reproduction 

 that are peculiarly prevalent: reproduction by spores, and 

 reproduction by gametes. In both of these methods the 

 offspring consists of a single cell, whereas the adult may 

 include millions of cells. Thus the ratio of the offspring to 

 the parent is in volume very small. This, as we have seen, 

 is much more economical to the parent, but it puts a long 

 period of immaturity and dangers before the offspring. 



In this situation the organism has two possible ways of 

 increasing the chances of enough offspring coming to 

 maturity. In the first place, it may produce a much larger 

 number of the single-celled offspring in order to overcome 

 the great fatality. Or, it may give especial care and 

 protection to the offspring and thus diminish the fatalities. 

 Indeed it may do both. It is in this care of offspring after 

 they are produced that the higher plants and animals excel. 



4. Parental Care of Offspring. You must not jump to 

 the conclusion that the care of young by parents is 

 necessarily conscious and deliberate. In man it is partly 

 so, but for the most part it is instinctive and unconscious. 

 It is none the less valuable on that account. The care of 

 offspring in some way or other is one of the most common 

 things in nature. That it is a good arrangement is 

 shown both by the wide-spread occurrence and by the 

 kind of success that comes to those species in which it 

 is best developed. Remember also that parental care 

 means sacrifice just as reproduction does. It is another 

 kind of sacrifice. 



