670 



ECOLOGY 



centimeters at the anterior end, while at the posterior end a correspond- 

 ing portion may decay. Thus the plant occupies a new position in the 

 soil each year, and if the advance is in lines, as in Polygonalum and in 

 Juncus ballicus, the phenomenon is known as linear migration. In 

 Juncus several erect shoots develop in a line each year, making linear mi- 

 gration very obvious, even to the casual observer. In Solomon's seal 

 {Polygonalum, fig. 983) a single erect shoot develops each year, and, as 

 it leaves a definite scar, it is possible to determine the age of the un- 



FiG. 981. — The basal portion of a 

 shoot of the ditch stonecrop (Penthorum 

 sedoides), showing the development of new 

 rhizomes; note that the upper rhizomes 

 (r, r') are more pronouncedly progeotropic 

 than are the lower rhizomes (r"); i, scale 

 leaves. 



Fig. 982. — An obliquely ascend- 

 ing rhizome of Juncus balticus, 

 illustrating the reaction of this 

 species to sand submergence; *", 

 scale leaves. — From Scholz. 



decayed portion of the rhizome; it is possible also to learn something of 

 the life conditions of each season on account of variations in the annual 

 increment. More commonly the radiation and branching of rhizomes 

 in all directions from the original center result when isolated in sym- 

 metrical colonies of ever increasing circumference. Sometimes (as in 

 the flags) the death of the older portions in the interior of such a colony 

 results in the formation of a ring, comparable to the " fairy rings " of 

 various fungi (p. 807), though continued branching is more likely to 

 keep the entire space occupied (fig. 984). 



The role of rhizomes. — The rhizome habit, perhaps more than any 

 other, facilitates the occupation of space by plants, especially because 



