STEMS 



671 



adventitious roots and erect shoots develop at various points on the 

 radiating stems. The older parts gradually decay, so that the branches 

 become isolated as separate plants; consequently there is an increase in 

 the number of individuals as well as in the space occupied. In this 

 phenomenon of vegetative reproduction, however, it is not a matter of 

 particular importance whether or not the actual number of individuals is 

 increased by isolation. The imp>ortant matter is the occupation of new 

 space, for in any case a rhizome colony or rhizome complex represents a 

 number of potential individuals, as 

 is well shown after the plowing of 

 a field partially occupied by such 

 plants (e.g. thecouch gra.ss, A gropyrum 

 repens) ; the rhizomes are dislodged 

 and broken and the scattered frag- 

 ments form new centers of migration. 

 The great advantage of rhizomes as 

 organs of propagation is due partly 

 to their horizontality, partly to their 

 elongation, and partly to the fact that 

 they are soil structures and thus are 

 able to invade regions already oc- 

 cupied. Seeds fall in numbers every- 

 where, but hundreds die where one 

 develops, because of the difficulty 

 of striking root in ground already 

 preempted; even plants with runners 

 propagate with difficulty where vege- 

 tation is dense. Rhizomes, however, 

 penetrate the soil of forests or of grasslands scarcely less readily than 

 that of open grounds. The advantage of the rhizome habit is well 

 illustrated in fields that have lain fallow; during the first year, annuals 

 usually dominate, but little by little rhizome plants and other perennials 

 increase their area, invading the space previously occupied by annuals, 

 until finally all of the latter are excluded. 



Fig. 983. — ^ A rhizome of the Solomon's 

 seal (Polygonatum biflorum); note the 

 tuberous enlargements of the rhizome 

 with the conspicuous scars is) left by 

 the fall of the erect stems of previous 

 seasons; r, adventitious roots; e. erect 

 stem of the current season. 



Often it is thought that plants exhaust the soil in which they grow, and therefore 

 that rhizomes are additionally advantageous in that the plant has a means of mi- 

 grating into new soil richer in food materials. However, it is now known that 

 plants rarely exhaust the soil, at least in nature. It might be supposed that migra- 

 tion is advantageous as a means of withdrawal from soil regions in which deleterious 



