102 COLLEGE BOTANY 



produce new individuals without the intervention of sexual or- 

 gans and some of them reproduce entirely or almost entirely 

 in this way. By this method new plants may be produced from 

 stems, bulbs, roots and occasionally from leaves. 



It is well known that many plants, such as lilies, tulips, hya- 

 cinths and onions reproduce by means of bulbs, which are modi- 

 fied stems (Fig. 19). Some of these bulb-producing plants also 

 reproduce by means of seeds, but others depend entirely on the 

 bulbs. The' growing of bulbs, especially of ornamental plants, is 

 one of the very important industries. 



Some plants, such as the strawberries, reproduce by "runners 

 (Fig. 15) or modified stems which grow along the surface of the 

 ground, take root and produce new plants, while others, such as 

 the mints, the couch or quick-grass and many others, reproduce 

 by rootstocks or rhizomes, which grow just "beneath the soil 

 (Fig. 17). Other plants, such as the potato, reproduce by means 

 of tubers (Fig. 18), which are fleshy underground stems. They 

 are propagated for agricultural purposes by cutting the tuber 

 in pieces. It is very difficult to destroy the rhizome type of 

 plants, because cultivation merely breaks these underground 

 stems in pieces and each fragment carrying a bud produces a 

 new plant; therefore, the number of "plants is materially in- 

 creased. Many other plants, such as blackberries and raspber- 

 ries, reproduce by "means of " suckers " or new plants which 

 grow directly from the lower part of the old stem. The stems 

 of many plants, especially the shrubs, will take root and grow 

 if they come in contact with the soil or become partly covered 

 with soil. Many trees and shrubs, especially the willows and 

 poplars, self-prune; i.e., drop the branches, during the growing 

 season (Fig. 75). These self-pruned shoots frequently become 



