CLASSES OF PLANTS 445 



pets of breeders. The seeds of the sunflower are useful as feed 

 for birds and poultry, but most of us find it too much trouble to 

 reach the kernel. The teasel was formerly used by the wool 

 w^orkers to tease out the raw fibers, but that work is now done 

 mostly by machinery. The thistle probably gets its name from 

 this former use of its close relative. The goldenrod has acquired 

 a bad reputation because many of the sufferers from hay fever 

 are found to be sensitive to the pollen of these plants ; but there 

 are probably many people who can become sensitive to almost 

 any pollen protein. 



SUMMARY 



1. While it is impossible to arrange all the known plants in a single 

 series from lower to higher, we can so arrange the plants in many of 

 the subdivisions ; and the main divisions themselves may be considered 

 as levels of development. In each branch the higher forms are dis- 

 tinguished by a greater specialization or division of labor among the 

 organs, or a greater special adaptation to particular conditions of living. 



2. In every main division of plants we find some that are useful to us 

 either directly or indirectly, and some that are harmful to us either di- 

 rectly or indirectly. 



3. Sometimes closely related forms may be both useful and harmful. 



4. Sometimes a single species may have qualities or materials that 

 are of use to us and other properties or materials that are harmful. 



5. There is no general rule for distinguishing poisonous plants from 

 those that are good to eat or harmless ; sometimes a plant may contain 

 both useful food and poison. 



6. Many plants that are useful in one region are nuisances in another. 



7. Plants that are useful at one period may become useless through 

 the development of cheaper or better substitutes, or through changes in 

 our way of living. 



8. Plants that are of no use may become valuable through the discov- 

 ery of their properties by scientific investigation or through the devel- 

 opment of new needs. 



9. Plants growing in one part of the world may come to be of value to 

 people in another part of the world. 



