THE EVOLUTION OF PLANTS 55 



favorable conditions prevail, their role in soil development, and in 

 moisture conservation. 



Spermatophytes. Seed bearing plants are the final step (thus far) 

 in the evolution of vegetation. The earliest to make any real progress 

 were low growing evergreens, which bore naked seed and depended 

 on the wind to carry clouds of fertilizing pollen as a prodigal and 

 somewhat inefficient but nevertheless effective means of reproduction. 

 These conifers were hardy specimens and could survive in cold, dry 

 climate and on infertile soil. They were rugged and still are. 



More recent, more specialized, and more dependent on a highly 

 developed and congenial environment are the flowering plants. Re- 

 production is most efficient, a relation having been established in many 

 cases with insects which do a precise job of pollination, and in return 

 secure nectar and excess pollen. The insect also assures itself (un- 

 knowingly, no doubt), of a new supply of food next year by aiding 

 new plants in getting started. There are exceptions; some flowering 

 plants have switched from insect to wind pollination, ragweed for 

 instance, and it will be agreeable to many people if this one changes 

 to some other less extravagant system. 



The growing and reproductive parts of the flowering, seed plants 

 are, as a rule, well protected. Through the winter the new plant life 

 is packed into tough, waterproof buds, which in many species open 

 only when daylight of a certain number of hours occurs. Others are 

 indeterminate, and bloom when physically developed. In either case 

 the male cell is protected in a pollen grain and the egg is deep within 

 the flower. 



The flowering plants are plastic and adaptable as a whole. They 

 are the most delicately organized, yet so much more efficient than 

 other plants that, like man, they have covered the earth. These 

 royalty could not exist without the lower forms which help prepare 

 the soil minerals, help maintain the water supply, and dispose of the 

 debris left when life departs. Each form of life is a wheel in the 

 mechanism of nature and its function is essential to a smoothly 

 working machine. A great deal of man's tinkering with this ma- 

 chine makes us think of the ten-year-old who "adjusted" his mother's 

 wristwatch with an icepick. 



Environmental Requirements. The patterns on the landscape as- 

 sumed by our 300,000 kinds of plants are determined to a large extent 

 by the nature of the plants themselves. In some cases the accident of 

 geographic barriers, such as mountains and oceans, may have its influ- 

 ence. Not all the plants which will grow on a piece of ground will be 

 found there, even in nature undisturbed. 



Having evolved in a specific environment, plants may be expected 

 to have some limitations as regards soil fertility, soil acidity or alka- 

 linity, light intensity and duration, growing season, temperature 

 range, ranfall, humidity, drainage, and associate plants and animals. 



