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, cudtngs, 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. 
