PLANT-BREEDING 317 



Such propagation is much more certain, for propagation 

 by seed introduces variations. Some plants are propagated 

 naturally in this way, as those with thickened underground 

 shoots (rootstocks, tubers, bulbs) or with runners (straw- 

 berry, etc.). 



Others are propagated by artificial methods. For ex- 

 ample, cuttings, often called slips, are pieces of the plant 

 that are found to be able to grow when put in the soil, as of 

 geraniums, grape-vines, etc. Even leaves may be used as 

 cuttings, as in the begonia; and cuttings of the potato tuber 

 are used in its propagation. Grafts are cuttings inserted in 

 plants ( 24) (Fig. 55), and it is common for the plant in 

 which a graft is inserted (stock) to differ from the plant that 

 is being grafted on it, securing among other things greater 

 hardiness and a saving of time; for example, it is common 

 to graft pears on quince stock. Budding is a variety of 

 grafting in which only buds from the desired variety are 

 grafted upon stocks. Grafting and budding are very com- 

 mon in the cultivation of tree fruits. Layering consists 

 in bending down a stem to the ground and covering it for a 

 short distance with soil; when roots strike into the soil from 

 a covered joint, the connection with the parent plant is cut, 

 and a new plant is thus obtained ( 23). This process is 

 common with such plants as gooseberries, blackberries, etc., 

 and resembles in a general way the natural method of 

 propagation by runners. 



179. Crossing. The artificial production of hybrids 

 ( 149) is used extensively to secure new varieties which 

 may be desirable. The process consists in removing the 

 young stamens from the flower to be operated upon; at the 

 proper time placing upon the stigma pollen from the de- 

 sired plant, and covering the flower or flower-cluster thus 

 pollinated with a gauze or paper bag to prevent the ap- 

 proach of any other pollen. The seeds thus obtained are 

 carefully collected and planted, and the new plants observed. 



