28 



THE ECOLOCxY OF STEMS 



Frequently the older parts die off each year about as rapidly as the 

 newer parts elongate and this, in the case of branched rhizomes, 

 results in the multiplication of individuals. This multiplication of 

 individuals is not so important, however, as the advance into new 

 territory. The fact that the rhizome is underground makes it pos- 

 sible for it to advance into an area that is alread.y occupied by plants 

 almost as readily as into one that is not so occupied. This is clearly 

 shown whenever a field is plowed and then left undisturbed for sev- 

 eral years. Such a field is practically always occupied first by 



Fig. 8. — A pine tree, showing excurrent type of brandling. 



annual plants, but within a few years the annual plants are crowded 

 out and replaced by perennial plants which have entered for the 

 most part by means of rhizomes. The conquest of such an area 

 through the agency of seeds would take much longer because it is 

 so difficult for the seeds to get down to the soil through a plant 

 cover. 



Runners are very much like rhizomes except that they grow 

 above the surface of the soil. They are not so common nor so effi- 

 cient as rhizomes, although some plants, such as strawberries, are 

 able to spread very rapidly by means of runners. Somewhat similar 

 to runners, at least in function, are such stems as those of the rasp- 



