232 ELEMENTARY STUDIES IN BOTANY 



position for vegetative propagation, and many prostrate 

 plants respond to the favoring conditions. As the prostrate 

 stem advances over the ground, roots develop from the nodes 

 and enter the soil, and a new plant is started, which becomes 

 independent by the death of the older parts or by separation 

 from them. In this way a plant may spread over the ground, 

 multiplying itself indefinitely. A very familiar illustration 

 is furnished by the strawberry plant, which sends out pecul- 

 iar leafless branches called " runners/' which " strike root " 

 at the tip and start new plants which become independent 

 by the death of the runners (Fig. 199). 



Prostrate stems illustrate the fact that nodes can pro- 

 duce not only leaves and branches, but also roots, if placed in 

 suitable conditions. Advantage is taken of this fact in the 

 common process of " layering," in which such stems as those 

 of blackberries and raspberries are bent down to the ground 

 and covered with soil, when the covered nodes strike root 

 and new plants are started. When any plants are propa- 

 gated by pieces of stem, known as " slips" or "cuttings," 

 as in grape culture, it is because nodes can develop roots 

 and thus make possible independent plants. The nodes 

 of some plants put out roots when not covered by soil or 

 even in contact with soil. For example, the erect stem of 

 corn sends out roots freely from the nodes near the ground, 

 and the poison-ivy and trumpet-creeper cling to supports 

 by tendril-like roots produced by the nodes. 



All that has been said emphasizes the importance of nodes, 

 which can produce leaves, stems, and roots ; and therefore 

 a single node from an old plant may make a new plant 

 possible. This is further emphasized in the method of 

 propagating potatoes, which are thickened subterranean 

 stems called tubers (Fig. 217). It is well known that when 

 a tuber is cut in pieces for planting, each piece must contain 

 one or more " eyes." An " eye " is a branch bud in the axil 

 of a minute scale-leaf. There is no special virtue in the eye 



