ROOTS AND RHIZOIDS 



503 



tinues to grow when transplanted, as in beets. In some plants, as in the 

 radish and the Windsor bean (Vicia Faba), when the tip is cut just below 

 the region, of active growth, the primary root continues its activity, ultimately 

 regenerating a new tip. If the root is cut above this region, further growth is 

 checked and lateral roots take its place, as described above. 



Adventitious roots. Adventitious roots (i.e. roots arising from stems 

 or leaves) reach their greatest development in plants with horizontal 

 stems (fig. 712; also figs. 978, 983). At first there is in such plants a 

 primary tap root and an erect stem, but soon a horizontal stem develops 

 beneath or just above the surface, whereupon adventitious roots issue 

 from the nodes, and the primary root and stem soon die. In bulbous 

 plants a number of roots of about equal size develop at the base 

 (fig. 991). If such a bulb is removed from the soil at maturity and 

 stored in a dry place, these roots die, but new adventitious roots 

 develop rapidly, when there is access to water. 



While adventitious roots usually are observed in plants with runners 

 or with underground stems, many other plants are 

 able to develop such roots upon occasion. When 

 long stems of the black raspberry bend over 

 until they touch the ground, adventitious roots 

 develop ; this habit is made use of in artificial 

 propagation, since a new shoot develops where the 

 stem strikes root. The same principle is utilized 

 in the artificial propagation of the rubber plant 

 (figs. 713, 714). If wet moss is tied about a 

 stem, root development at that point is soon in- 

 cited, whereupon the stem is cut off below the 

 moss and placed in soil. 1 



Various swamp plants, such as the reed and the swamp 

 loosestrife (Decodon verticillatus'), strike root where the 

 stem comes in contact with wet soil, and the stems of 

 various willows and dogwoods (as Cornus stolonifera) 

 develop roots in abundance when placed in water or 

 in moist soil (fig. 715). Such behavior is of immense ad- 

 vantage in the case of plants that are partly buried by 

 accumulating humus or sand, for as fast as the stems 

 are buried, new adventitious roots appear at higher levels, veloped at the nodes. 



FIG. 715. A winter 

 shoot of a willow (Salix), 

 which has been placed 

 in water; note the adven- 

 titious roots that have de- 



1 A remarkable development of adventitious roots is seen on the trunks of some tree 

 ferns (as Dicksonia), where they may appear in such abundance as to enclose the trunk in 

 a spongy mass. 



