THE EVOLUTION OF PLANTS 53 



chemically uninhabitable, or reduce the oxygen by an excess of decay- 

 producing microbes. Remedying such blunders is a problem for sci- 

 entists, educators, economists, and statesmen. It is just as much a 

 problem for each individual citizen. The success, or lack of it, in 

 reaching a solution is a good guage of civilization. 



Terrestrial plants seem to have evolved along the seacoasts where 

 tides come and go. Certain variant or mutant plants found them- 

 selves able to live through the low tide periods. Through the leisurely 

 eons of geologic time, mutation after mutation slowly produced sea 

 weeds which could extract minerals from a thicker mixture of soil 

 and water, endure the greater and more rapid temperature fluctua- 

 tions of land and air, and conserve moisture between wet periods. The 

 change was not too great because the evolution probably took place in 

 soupy, muddy areas under high atmospheric humidity. Nevertheless, 

 land plants found a much greater complexity and variability of envi- 

 ronment and higher order mutations had greater opportunity to 

 survive. 



Ill KINDS OF PLANTS 



The plant kingdom may be divided into four great groups. The 

 plants of each group have some factors in common, yet embrace a 

 wide variety of species and sub-species. For the benefit of those who 

 are not familiar with these or to refresh a dim memory we will define 

 them briefly. The four groups, or phyla, are (1) thallus plants, (2) 

 mosses, (3) ferns, and (4) seed producing plants. 



Thallophytes. The Thallus, or soft bodied, plants are historically 

 the oldest and simplest. They may have one cell, or more, such as 

 bacteria, algae, fungi. By their simplicity of structure and by adap- 

 tation, many of them can live in a wide range of environments. Their 

 original home is the sea and there many of them are found today, 

 cither as part of the plankton pasture, on lighted bottoms, on other 

 plants, on animals or on decaying matter. Some thallophytes have 

 made the grade on land, enduring periodic dryness that would kill 

 more highly developed plants. Most of the group are parasites on 

 living forms, or saprophytes on dead ones. They are agencies of 

 disease and decay, both of tremendous importance to man. We can- 

 not ignore these unspecialized plants without roots, stems or leaves. 

 They are part of the challenge in managing the total environment to 

 sustain a permanent civilization. 



Bacteria and fungi which produce disease in man, other animals, 

 and plants, and which ruin valuable organic property such as food, 

 clothing, and leather goods, must be controlled. 



Bacteria and fungi which return humus to soil, break down min- 

 eral nutrients for plant use, manufacture nitrates, promote a granu- 

 lar soil, and render other services, must be encouraged. Many pres- 

 ent day practices on the land, such as the failure to add organic 

 matter, and the type of mismanagement which produces erosion, 

 discourage these valuable life forms. 



