spores which escape in due time. As in the moss, we will 

 begin with the spore. 



a. Germination of Spore and Formation of the 

 Gametophyte. The spore germinates by sending 

 out a tube as in the moss, but, instead of forming 

 a long-branched filament, it divides in all planes and 

 makes a small, flat, heart-shaped green body known 



as a prothallium. This is not likely to be seen by 

 the student unless his attention is called to it. It 

 will grow only in moist places. It lies flat, has 

 delicate root-like structures (rhizoids) on the under 

 as a prothallium. This is not likely to be seen by 

 side, and a notch at the growing end. This 

 prothalliurh is the gametophyte generation. 



b. The Formation of Gametes anf Fertilization. The 

 green upper side of the gametophyte receives the 

 light, and its cells manufacture foods as do the 

 leaves of the higher plants. At certain points on 

 the under surface two kinds of bodies are formed, 

 with exactly the same names and the same 

 functions as those that grow at the top of the leafy 

 moss plant. The archegonium is much the same 

 shape as in the moss, but the antheridium is more 

 flat and rounded. The sperms, as in the moss, must 

 have moisture in which to swim to the archegonium. 

 The egg at the bottom is fertilized just as in the 

 moss. 



c. Development of Sporophyte and Formation of 

 Spores. The fertilized egg divides, and the four 

 cells formed by the first divisions develop into the 

 parts of the new plant. Unlike the moss embryo, 

 the fern embryo does not depend long for 

 nourishment on the small gametophyte. As soon as 

 its growth causes it to break out of the small cavity 

 of the archegonium, it begins to develop roots that 

 take hold of the soil and leaves that manufacture 

 food for further growth. Furthermore, instead of 

 stopping with a small seta and capsule, growth 

 continues in the fern sporophyte until we frequently 

 have a rather large plant with roots and numerous 

 handsome leaves. Later in the season these leaves 

 produce on their under surfaces the spores with 

 which we started. 



5. Comparison of Moss and Fern. It will be seen from 

 the above that the moss and fern follow very much the 

 same course of development in their life cycle: spore 

 germination; development of the plant that bears gametes; 

 formation of gametes; fertilization; development of the 

 embryo into another generation of the plant wholly 

 different in appearance from the gametophyte; the 

 development of spores by this new generation 

 (sporophyte) ; the escape and germination of the spores. 



It will be noticed in both kinds of plants that when the 

 spores germinate they produce a plant that forms gametes 

 and not spores; and when the gametes unite they start a 

 plant that develops spores and not gametes. We have in 

 both a regular alternation of two different kinds of 

 individuals in one life cycle. 



There are also some interesting differences: 



Moss Fern 



1. The gametophyte is a 1. The gametophyte is a 



leafy plant; the most poorly developed part; 



conspicuous part of the the poorest part of the 



cycle; the part called the cycle; the part called the 



"moss-plant." prothallium. 



posed of foot, seta, and 

 capsule. It is called a 

 sporogonium. 



leaves and roots and is 

 the real "fern plant." 



2. T h e sporophyte is 

 wholly parasitic on the 

 gametophyte, and develops 

 only slightly, being com- 



2. The sporophyte soon 

 becomes independent of 

 the gametophyte, fixes 

 itself to the soil, develops 



CHAPTER FOURTEEN. 

 ALTERNATION OF GENERATIONS. 



1. Summary of Reproductive Methods. We may class 

 the methods of reproduction that have been studied as 

 follows: 



1. Those that occur without union of gametes, as in 

 fission, budding, spore formation, etc. (non-sexual). 



2. Those that involve union of gametes, whether 

 similar or unlike (sexual). 



We have studied many forms of plants and animals in 

 which both non-sexual and sexual reproduction are found. 

 For example, hydra buds and also produces eggs and 

 sperms; many of the algae reproduce both by swimming 

 spores and by gametes. In the forms referred to, however, 

 we must note one important fact: the final organisms 

 produced by the two methods were alike. We could not 

 tell by looking at an individual hydra whether it had been 

 formed as a bud or had come from a fertilized egg. 



2. Alternation of Generations. There is another step in 

 the story which we must now master. It often happens 

 that this reproduction by union (sexual) and reproduction 

 without union (non-sexual) occurs in the same species, in 

 regular alternation, as we have seen in mosses and ferns. 

 Furthermore, the individual that results from non-sexual 

 reproduction is different from the individual formed by 

 the union of gametes, usually so different that no one 

 would take them as belonging to the same species. This 

 is called alternation of generations. If we illustrate by 

 letters, we may describe such, an alternation in this way: 

 An individual A reproduces non-sexually, and when the 

 offspring become mature they are not As, but Bs. B, when 

 mature, develops gametes which unite, and the fertilized 

 eggs develop into organisms that are not like their parent 

 B, but like the grandparent A. A by budding or spore- 

 formation produces B; B, in turn, by sexual union, 

 produces A. 



3. The Moss and Fern Illustrate this Alternation. 

 What has been described in the preceding section is 

 exactly the thing that happens in the mosses and ferns. 

 The small prothallium in the fern, which has been called 

 the gametophyte, has the two kinds of gametes. These 

 unite, start a new plant, which is the large fern plant that 

 grows to be very different from the little microscopic 

 prothallium. This large plant is the sporophyte generation. 

 It has a period. of growth, and when it becomes mature it 

 cannot in any way produce eggs and sperms, but produces 

 great numbers of spores without any sexual union 

 whatever. We have seen that these spores germinate into 

 the gametophyte. The sporophyte is one generation and 

 the gametophyte is another. These generations regularly 

 alternate. 



In the moss we have exactly the same facts and in the 

 same order. In the fern both generations live independent 

 lives and the sporophyte is the better developed generation. 

 In the moss the gametophyte is the more important 

 generation and the sporophyte is parasitic on it. One 

 question about the moss may very properly be raised by 



