54 



ECOLOGY 



supplementing the original root system. Some stems, such as those of pines and 

 oaks, are unable to develop roots in this manner, a fact that may account for the 

 early death of these trees when partly buried, in contrast with the phenomenal 

 success of poplars and willows under similar conditions. 



Adventitious roots occur less frequently on leaves than on stems. The walking 

 fern (Camptosorus rhizophyllus) strikes root at the leaf tip, much as does the rasp- 

 berry at the stem tip, and in a similar way gives rise to a new plant. While not 

 particularly common in nature, leaf roots are induced readily in a number of 

 species when the leaves are removed from the parent plant and placed in contact 

 with moist soil. In this way gardeners propagate various plants, such as Begonia, 

 Peperomia, and Sansevieria (figs. 933, 934). 



Probably moisture is the chief factor determining the development of adventitious 

 roots, not alone in water, but also in soil, and in instances like those noted in the 

 rubber plant and in the tree ferns. However, given equal moisture, adventitious 

 roots appear to develop most on darkened and on under surfaces (fig. 933); prob- 

 ably contact with solid bodies also is an important stimulative factor. 



Root contraction. When a " stemless " rosette plant, as the rock 

 cress or dandelion, grows in a crevice, the position of the rosette in rela- 

 tion to the surface is year 

 by year the same (figs. 716, 

 717). The very short stem 

 elongates slightly each year, 

 and the root pulls the plant 

 by that much into the soil, 

 so that the rosette remains 

 at a constant level. This 

 phenomenon is called root 

 contraction; the mechan- 

 ism of the process is not 

 fully understood, but its 

 advantage to the plant is 

 very evident. Sometimes 

 the contractile region of the 

 root becomes less than two thirds its original length (the shortening 

 may exceed fifteen millimeters in Arisaema Dracontium)', after con- 

 traction the root exhibits transverse folds, and observation shows that 

 the contractile tissue is chiefly the cortical parenchyma (fig. 720). 



In white clover and in similar plants, erect or ascending stems are pulled down 

 by the adventitious roots, which develop at the nodes and hold the stems tightly to 

 the soil. The varying soil levels which characterize underground stems of different 

 species often are reached through stem activities (see figs. 718, 719). As crevice 



FIGS. 716, 717. Diagrammatic soil sections, 

 illustrating the lowering of a rosette plant (Arabis 

 lyrata) from year to year by root contraction: 716, 

 a two-year-old plant with rosette leaves (r) at the 

 surface, and with remains of the previous rosette (r r ) 

 deeper down; 717, a three-year-old plant, which has 

 remains of a rosette two years old (r"), now deeply 

 buried; these diagrams also illustrate multicipital 

 stems (p. 676). 



