THE ANGIOSPERMAE : STEMS 



931 



broader than the stem itself. In many cases the wings are clearly downward 

 prolongations of the leaves or stipules, but in a few cases, notably in Genista 

 sagittalis (Fig. 915), they are due to the expansion of the stem tissues. Such 



^ 



Fig. 915. — Genista sa^it talis. Flowering 

 shoot showing winged stems. 



cases are distinguishable from phylloclades by the existence of a normally 

 organized stem, to which the expansions are attached (Fig. 916). 



4. Runners. These are plagiotropic stems which grow on the surface of 

 the soil, often for considerable distances (Fig. 917). They are a potent means 

 of vegetative propagation (see Volume IV), and they form a natural transition 

 to rhizomes (see p. 933) with which they may sometimes be interchanged, 

 as in Phragmites. Normally the Reed produces rhizomes below the surface 

 of the mud on which it grows, but if the ground dries up runners are 



