524 USEFUL PLANTS AND PLANT PRODUCTS 



shipment. The refuse from beet sugar manufacturing establish- 

 ments is used in a wet condition for cattle feeding, and is also 

 dried and shipped. 



Some seeds not eaten by man are highly valuable when fed 

 to the lower animals. Acorns and beechnuts, for example, in 

 some of the wooded portions of the southern Middle States, fur- 

 nish a considerable part of the subsistence of droves of hogs. 



4. PLANTS USED AS FERTILIZERS 



501. For centuries the advantage of plowing under growing 

 crops as a means of enriching worn-out land has been well 

 recognized. It is only very recently that the exact significance 

 of this process has been understood. Even now the details are 

 not so fully worked out that we know just what crop will yield 

 the best results for every variety of soil and climate ; but in a 

 general way it is established that leguminous plants are the 

 best for this purpose on account of the power which their root 

 tubercles have of utilizing the nitrogen of the atmosphere 

 (Sec. 256). Various clovers and alfalfa are the crops most 

 commonly employed. 



5. PLANT PRODUCTS USED IN MANUFACTURES 



502. Under this head there is only space to mention a very 

 few of the vegetable substances used in manufacturing processes, 

 most of them on account of their chemical properties. 



Dyeing by means of vegetable coloring matters is far less 

 important than it was before the introduction of the artificially 

 prepared aniline colors. These are so powerful that it is more 

 economical to use them, but they do not give soft shades. Val- 

 uable dyes, however, are still obtained from a considerable num- 

 ber of plants. Many of these belong to members of the pea 

 family, which furnishes Brazil wood (red), logwood (red, purple, 

 and black), camwood (red), indigo (dark blue). 



