CHAPTER XVII 

 FLOWERS AND THEIR USES 



269. Flowers and the Relationships of Plants. — Every 

 one knows that there are many kinds of flowers, differing 

 from one another in size and color, in the shape and arrange- 

 ment of their parts, and in the way in which they are borne 

 upon the plant. One reason why much study has been de- 

 voted to flowers is that the structure of a flower tells us more 

 about the relationships of the plant that bears it than does 

 the structure of any of the vegetative parts — stem, leaves, 

 or roots. This is because the vegetative parts change oftener 

 and more rapidly in the course of the evolution of plants 

 than flowers do. 



The conditions under which plants live are continually 

 though slowly changing, and they have been changing for 

 ages. If a region that has been low and wet becomes high 

 and dry, the species of seed plants that live in that region 

 must either die out or undergo changes in their roots and 

 leaves that will adapt them to the new conditions. The 

 same is true if land that has been high and dry becomes low 

 and swampy ; or if the formation of a new mountain range 

 affects the direction of the winds and the amount of rainfall 

 in the country on either side. Changes such as these in 

 the earth's surface have compelled the greatest variety of 

 changes in the vegetative parts of plants, and so have re- 

 sulted in the development of many new species. But a 

 change in the conditions surrounding a plant is not so likely 

 to make changes necessary in its flowers, which are not so 



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