62 THE ARRAY OF LIVING ORGANISMS 



Exercises XII and XIII 



Guidelines 



(1) Alternation of haploid gametophyte and 

 diploid sporophyte generations. 



(2) Progression from gametophyte-dominance 

 (mosses, liverworts) to sporophyte-dom- 

 inance. 



(3) Emergence from water to land : vasculari- 

 zation. 



(4) Mosses, liverworts, and ferns as the 

 amphibia among plants. 



(5) Development of dry fertilization in con- 

 ifers and flowering plants. 



A major difference between plants and ani- 

 mals, one which runs throughout the entire 

 plant kingdom and extends back among the 

 colonial algae, involves their reproductive habits. 

 Animals are almost invariably diploid (the 

 nuclei of their cells contain /?fl/>.y of chromosomes, 

 forming a double set) except for the mature germ 

 cells (gametes: eggs and sperm), which are 

 haploid (i.e., each contains a single set of 

 chromosomes). 



In contrast with this, plants alternate a diploid, 

 spore-bearing (sporophyte) generation with a 

 haploid, gamete-bearing (gametophyte) genera- 

 tion. It is true that in higher plants the gameto- 

 phytes, male and female, are very small, and live 

 upon the diploid sporophyte, which forms the 

 main body of the plant. Yet in lower plants these 

 relations are reversed, and all plants display the 

 basic pattern of alternation of generations. This 

 is how it works: 



(1) Diploid sporophyte by meiosis (cell divi- 

 sion with reduction from double to 

 single chromosome number) yields hap- 

 loid spores. 



(2) Haploid spore by ordinary cell division 

 (mitosis) yields a haploid gametophyte, 

 bearing haploid gametes. 



(3) Fertilization of an egg by a sperm restores 

 the diploid number of chromosomes, and 

 by mitosis yields a new diploid sporophyte. 



The forerunners of modern plants were un- 

 doubtedly aquatic. They had to face neither the 

 problem of conveying food and water over rela- 



tively long distances, as must be done in the 

 larger land plants, nor providing devices by 

 which the germ cells could find one another, 

 which is no problem in water. 



Mosses and liverworts, most of which made 

 the transition from water to land, have compro- 

 mised with both these problems. They are non- 

 vascular (i.e., lack conducting vessels), and hence 

 are restricted to a small size, which keeps them 

 close to the ground. Also, at the time of sexual 

 fertilization they must manage to collect enough 

 water for the sperms to swim to the eggs. In 

 these plants the gametophyte is the dominant, 

 free-living generation; it is what we mainly see 

 as the plant. The sporophyte is a relatively small 

 structure that remains permanently attached to 

 the gametophyte. 



With the evolution of specialized tissues to 

 overcome the difliculties of terrestrial living, 

 plants came to cover almost all the land masses. 

 The primary step in this development was the 

 evolution of vessels to conduct water and dis- 

 solved materials throughout the organism. This 

 step is so important that the plant kingdom has 

 traditionally been divided into two subking- 

 doms: the nonvascular thallophytes (algae, 

 fungi, bryophytes) and the vascular tracheo- 

 phytes, the higher land plants. The tracheo- 

 phytes have also developed further specialized 

 tissues, leaves, roots and stems, which have aided 

 in the colonization of the land. The simplest 

 tracheophytes, the psilopsids, club mosses, and 

 horsetails, most of which are now extinct, dis- 

 play the beginnings of all these developments. 



In all the tracheophytes the sporophyte is the 

 dominant generation; it is what we see as the 

 plant. In ferns, the gametophyte is still free- 

 living, though reduced to a very small size. In 

 the more advanced groups the gametophytes are 

 represented by only a few cells. 



The leafy plants (Pteropsids) have diverged 

 in the course of their evolution to form three 

 large groups: the ferns, conifers, and flowering 

 plants. All ofthem have well-differentiated roots, 

 stems, and leaves. The ferns, however, have still 

 not won freedom from one condition of aquatic 

 life; they still need water in which the sperms, 



