THE ANGIOSPERMAE : STEMS 



935 



soil it requires protection, which is afforded in one of two ways ; either by 

 smooth, hard and pointed scales which provide a boring point, as in Ammophila, 

 or by the folding back of the apex into a crook, of which the curved portion 

 is the part which actually penetrates the soil, as in Mercurialis perennis. 

 These crooks persist until the apex grows up into the light, after which they 

 unfold. If they are covered from the 

 light they do not unfold, even after they 

 have emerged from the soil. 



Rhizomes are nearly always branched 

 sympodially, owing to the orthotropic 

 development of the apex. Lateral 

 branches spread out plagiotropically in 

 the same horizontal plane as the original 

 rhizome, no matter from which side of 

 it they arise. The plagiotropic growth 

 of rhizomes is not, however, invariable. 

 Apart from the exceptional cases of 

 upwardly directed shoots formed as a 

 regulatory measure in rhizomes too 

 deeply buried, there are some instances 

 of the reverse, namely of direct down- 

 ward growth of a rhizome. These are 

 chiefly in Yucca, Cor dy line and Dracaena, 

 shrubby members of the Agavaceae. The 

 downwardly directed rhizome normally 

 functions as a storage organ pure and 

 simple, but if flowering or any accident 

 terminates the growth of the aerial shoot, 

 branches spring from near the base of the 

 rhizome, which become new aerial shoots. 

 The apex of the rhizome does not itself 

 develop thus and will only grow into an 

 aerial shoot if it is turned upside down. 



Although most rhizomes are well 

 provided with adventitious roots, there 



are a few cases in the Orchids, e.g., Neottia (Fig. 921) where no roots occur 

 and the closely branched rhizome system itself functions as an absorbing 

 organ {cf. Psilotum, Fig. 619). 



Most rhizomes are perennial structures, but many examples occur among 

 herbaceous plants {e.g., Aster) in which a crop of short rhizomes is formed 

 round the base of each aerial shoot. They grow only for one season and 

 are next season replaced by a fresh crop, springing from each new shoot, 

 while the old rhizomes rot away. Such plants occupy a limited area of ground 

 with extreme density. 



The term stolon is often to be found in older books, applied indiscrimin- 

 ately to various kinds of plagiotropic stems which root adventitiously. It is 



Fig. 921. Neottia nidus-avis. — Flower- 

 ing plant showing the densely 

 branched underground rhizome 

 system. 



