PLANTS 



39 



in dealing with life histories the dynamic relations are necessarily- 

 emphasized. 



Propagation has been the outcome of the progressive modification 

 of the shoot, partly through a changing relation to ecial factors and 

 partly through the differential storage of food for the buds. It has 

 also been deeply influenced by the nature of the shoot and especially 

 the bud. Out of this interplay have sprung a number of well-known 

 propagules widely employed by the gardener for artificial multiplica- 

 tion. These range from leafy shoots, modified only to the extent of 



It*''. ^- 



** ««»*^^iififi 



Fig. 1. — Propagation by root stocks and consequent migration of Co)tvolvulus 

 soldanclla on fore dunes of the strand ; southern Cahfornia. (Photo by Edith 



Clements.) 



developing roots where the tip or the nodes touch the ground, as in 

 the stolon, to types so transformed as to be recognizable as shoots 

 only by the presence of buds, such as corms. Between these lie off- 

 sets, runners, scaly and fleshy rhizomes, tubers, caudexes and bulbs 

 (Fig. 1). 



The nature of the propagule has a direct bearing upon other fea- 

 tures of the life history, such as aggregation, migration, duration, and 

 competition. AVhen the shoot extends in a horizontal direction, as in 

 the scaly root stock of quackgrass or the runner of the strawberry, 

 migration, though slow, is definite and assured. When it is short or 



