312 



Science of Plant Life 



the vast majority of our herbs. The poplars, oaks, hickories, 

 maples, and elms, and the chestnut, beech, and ash, largely 

 make up the Deciduous Forest of the eastern United States, 

 which dominates the area east of the Great Plains between the 

 upper Lake region and the Gulf coastal plain. 



The mustard family. This family includes the mustards, 

 cabbages, radishes, turnips, and cresses. It is of great 

 economic importance. The flower is characterized by four 

 petals placed at right angles in the form of a cross. The 

 development of cultivated plants from wild species is nowhere 

 better exemplified than in the case of the cabbage. The small 



wild plant from which it was 

 derived produced neither 

 heads nor swollen stems. 

 From it, by the selection of 

 mutations, have been devel- 

 oped several distinct forms of 

 cabbage and also cauliflower, 

 Brussels sprouts, kohl-rabi, 

 kale, and coUards. These 

 differ so much in appearance 

 that one would scarcely guess 

 that they had a common wild 

 origin. The histories of cul- 

 tivated plants afford many ex- 

 amples of far-reaching changes 

 in the forms of plants, brought 

 about in the same way. 



The rose family. This 



Fig i8s. Wild rose, and a vertical section f^^^j jg characterized by 

 of the flower. Most of the important tree •' _ ■' 



and shrub fruits belong to the rose family. flowerS with five petals, five 



