668 



ECOLOGY 



stems may originate, while in the former, foliage leaves and even flowers 

 (as in some violets) issue directly from the rhizome. In some species 

 of Viola and Polygala underground flowers develop from rhizomes 

 (fig. 1191). 



The reactions of rhizomes to changes of soil level. — The horizontaUty 

 of rhizomes has led to the general conception that they are diageotropic 

 organs, as probably is the case under ordinary conditions. How- 

 ever, rhizome ac- 

 tivities are too 

 complicated to be 

 accounted for 

 solely by diageo- 

 tropism. The un- 

 derground stems 

 of most species 

 have a definite 

 position in the 

 soil, varying from 

 a considerable 

 depth, as in Equi- 

 setum, Asparagus, 

 and many xero- 

 phytes, to a slight 

 depth, as in Jun- 



cus balticus and many swamp plants and mesophytes. When a rhizo- 

 matous plant is transplanted, it grows up or down to its specific 

 soil level, thenceforth growing horizontally. If soil is added, the 

 rhizome begins to ascend, sometimes almost vertically, as when Juncus 

 balticus is submerged by a dune (fig. 982); if soil is removed, the 

 rhizome descends. Thus a rhizome grows parallel to the soil surface; 

 this phenomenon is known as maintenance of soil position and is said 

 to illustrate the law of levd. The cause of this extraordinary behavior 

 is complex but it is believed that the nature of the geotropic reaction 

 varies with the depth. Experiments on Polygonalum show that the 

 distance separating the rhizome from the place where the aerial shoot 

 emerges into the light is the chief depth-determining factor; if the 

 aerial shoot is obliged to pass through a darkened layer of air, the 

 rhizome ascends, precisely as if a soil layer of equal depth were added. 

 Variations in the soil moisture or in the oxygen content of the soil also 



Fig. 978. — The horizontal underground stem (rhizome or 

 rootstock) of a false Solomon's seal {Smilacina siellata) ; note the 

 erect stalk of the current season (e) and the bud (&) which gives 

 rise to a similar erect shoot the following season ; note also the 

 lateral branch (6'), the beginning of another potential plant; r, 

 adventitious roots. 



