/To establish themselves in a barren region tough seed-plants must be able 

 to push their roots down rather quickly. They usually have harsh skin, often 

 prickly surfaces or hairy coatings, and can endure extreme changes in moisture 

 and in temperature. Such "pioneers" are able to make a living in rather un- 

 promising conditions. Through this very growth, however, these pioneers 

 change the surroundings. The roots break up the soil and make the latter fit 

 for more tender plants. The dead leaves falling to the ground make a blanket 

 that retains moisture: now the earth does not dry so quickly after a rain. Or- 

 ganic matter slowly accumulates and gets into the soil. In the shade of pio- 

 neers the seeds of more tender plants can get started. In time these early set- 

 tlers change the soil, and the climate close to the ground. They have made an 

 environment suitable for other species of plants. They have provided also a 

 setting for insects, worms, and bacteria and other nongreen species. 



The first animals to arrive in such a situation may find little to attract 

 them or to hold them. In time, however, food becomes available for more 

 kinds of plant-eating species. The remains of dead plants and animals supply 

 conditions suitable for scavenger animals like certain kinds of worms and in- 

 sects, and for decay organisms — bacteria, yeasts, fungi. Roaming birds and 

 other animals act as carriers — whether they remain or pass on. They bring 

 new kinds of seeds, as well as worms, protozoa, and insect species small enough 

 to take the ride, whether inside or outside the bodies of the larger forms. 



Changing Population The pioneer seed-plants are not merely tough in 

 relation to the physical and chemical conditions. They must also be self- 

 sufficient for pollenation. Or they must at least be able to get along with wind 

 and gravity, and not depend upon insects or birds. After insects arrive, there 

 may be a chance for the more sophisticated species of plants that do depend 

 upon insects. In much the same way, seeds of legumes (plants of the bean 

 family) may get started; but unless the right kinds of soil-bacteria are also 

 present, they will not be able to establish themselves (see pages 149, 151). 



The composition of a living population is thus constantly changing. Some 

 species become relatively more numerous. There are constantly new arrivals. 

 Some of the new settlers expand rapidly. Some of the early settlers gradually 

 disappear: they are crowded out, or they die out. In some cases, plants and 

 animals take on new patterns of living. A plant whose ancestors lived on moist 

 soil has now leaf habits and root habits that enable it to live in the drier region. 

 Or an insect whose ancestors lived for generations on a particular species of 

 plant takes to a different diet. But most species apparently make no experi- 

 ments unless driven by "hunger". 



The Climax Community^ Plants of diflferent species are constantly 

 competing for the limited amounts of water and of minerals in the soil. Some 

 compete for sunshine, although others thrive in the shade of their taller 



iSee No. 4, p. 576. 

 568 



