82 PEACTICAL BOTANY 



74. Stems as sources of animal food. The life of men and 

 of many species of the lower animals is largely sustained by 

 vegetable food obtained from the stems of plants. Cane sugar 

 and maple sugar are respectively derived from the stem of 

 the sugar cane and of the maple tree. The sugar maple is 

 tapped for its sap for sugar-making in early spring. The flow 

 of sap is most abundant during moderately warm days suc- 

 ceeding freezing nights. A single tree usually yields from 

 30 to 50 or more quarts of sap, from which 3 or 4 pounds of 

 sugar can be made. One tree has, however, been known to 

 yield 23 pounds of sugar in a single season. Asparagus, 

 cabbage, and a few other vegetables consist of aerial shoots. 

 Sago is made from the starchy pith of East Indian palms and 

 West Indian cycads. Potatoes, onions, and Jerusalem arti- 

 chokes are well-known examples of underground stems used 

 as food. Many familiar animals such as rabbits (more prop- 

 erly called hares), deer, and moose live largely by brows- 

 ing on the twigs of trees and shrubs. In pioneer times it was 

 sometimes necessary to feed to horses cottonwood and other 

 twigs in winter for lack of hay. Young cornstalks with the 

 leaves (corn fodder) form an important article of horse and 

 cattle food, and the preparation of fermented cornstalks known 

 as ensilage is widely used. The stems of prickly-pear cactus 

 (Fig. 65) deprived of their thorns (or of thorn less varieties 

 of this cactus), are used as food for cattle in the semi-desert 

 regions of the Southwest. 



75. Reproduction by portions of the stem. The number of 

 seed plants which are naturally reproduced by means of por- 

 tions of the stem is very large, and there are many others 

 which are artificially propagated by this means. Some of the 

 principal varieties of reproduction by pieces of stem or special 

 shoots for the purpose are : 



(1) By aerial bulblets. 



(2) By underground bulbs, rootstocks, tubers, and so on. 



(3) By offsets, stolons, and runners. 



(4) By broken-off branches (Sect. 60) or cuttings ("slips"). 



