THE ANGIOSPERMAE : STEMS 



933 



produces adventitious roots and becomes independent, for the internodes 

 die off in winter. 



Runners are commonest in herbaceous plants, but some shrubs also 

 produce them, such as Lonicera japonica and Rhus toxicodendron (Poison 

 \yy). 



Modifications of Underground Shoots 



I. Rhizomes. These are plagiotropic underground stems of the most 

 varied kind. They are found in a vast number of plants. In many species 

 they are thin, tough and rapid in growth, as for example, in Ammophila 

 arenaria, Carex arenaria (Fig. 918), Convolvulus arvensis and Agropyron 



■»'fa. 



Fig. 918. — Carex arenaria. Apical portion of a rhizome bearing scale leaves and 

 adventitious roots. Axillary branches grow upwards from the nodes and bear 

 the foliage leaves. 



repens. Others have relatively fleshy, though rapidly growing, rhizomes, as 

 Tiissilago farfara, ConvoUaria majalis (Fig. 919), Mercurialis perennis, Aego- 

 podium podagraria and Mentha spp. A third group of rhizomes are fleshy 

 and stumpv and serve more for storage than for spreading ; such are the 

 rhizomes oi Iris germanica,Polygonatiim multiflorum (Fig. 920) znd Symphytum 

 tuberosum. Of this latter class a number are dorsiventral, producing leaves 

 on their upper sides and roots only on their lower sides, besides being often 

 flattened in section. In Xuphar lutea and Xymphaea alba the dorsiventrality 

 only appears in old rhizomes and is conditioned by light, as they grow on the 

 bottom of ponds. If they are buried in the mud they grow out into radial 

 shoots which turn upwards until they reach the light, when dorsiventral 

 growth is resumed. 



