STEMS 



673 



ground, rooting copiously at the fre- 

 quent nodes (fig. 712). Also to be 

 classed with runners are such stems 

 as those of Decodon and Rubus which 

 bend over and take root if the tip 

 comes into contact with moist soil. 

 Prostrate stems differ from runners 

 in not taking root, though stems 

 classed as prostrate (Arclostaphylos, 

 Juniperus horizontalis, fig. 984) fre- 

 quently develop roots under favor- 

 able conditions. When the stems 

 of Decodon or Rubus bend over and 

 root at the tip, probably it is because 

 there is not sufficient mechanical 

 tissue to hold the stems erect. Pos- 

 sibly the same is true of many 

 runners, since they often appear 

 phophototropic at the outset. In 

 any case, the development of ad- 

 ventitious roots is likely thenceforth 

 to cause a downward pull on the 

 older parts of the runner. Ne- 

 phrolepis is somewhat unique in 

 possessing both rhizomes and run- 

 ners; the rhizomes develop first 

 and give rise freely to adventitious 

 roots. The runners are peculiar 

 leafless organs without a conspicuous 

 role; if they are removed, some of 

 the rhizomes develop into aerial 

 shoots. 



The advantages of runners. Run- 

 ners compare favorably with rhi- 

 zomes as organs of vegetative re- 

 production, as may be seen in the 

 development of a colony of straw- 

 berries or of white clover. They 

 commonly elongate more than do 



985 



FIGS. 985-988. Satureja glabra: 985. 

 the basal portion of a plant, showing a 

 runner (s) in which all of the leaves appear 

 to issue from the upper side, because of 

 stem twisting and petiole curvature; at the 

 nodes adventitious roots (r) issue from the 

 under side of the runner, fastening it to 

 the ground ; note the unfastened ascending 

 tip (/); if the runner is severed at a. a', or 

 a", the rooted portions develop into in- 

 dependent plants; 986. a short, broad leaf, 

 characteristic of a runner, developed 

 especially in the autumn, and remain- 

 ing over winter; 987, an elongated, nar- 

 row leaf, characteristic of the erect stem; 

 988, a short, narrow leaf, characteristic 

 of the floral region : 986-988 equally mag- 

 nified. 



