THE SEEDLING 25 



ward. With the appearance of the lateral rootlets there can, 

 of course, be no further elongation of that part of the radicle 

 or tap root from which they spring, since this would result only 

 in the destruction of the branch roots or a doubling of the tap 

 root. Observation of a marked primary root shows, in fact, 

 that elongation takes place only near the tip. The subsequent 

 development of the root system is simple enough. The main 

 branches increase in diameter, and, as they push out farther 

 into the soil, give off numerous smaller branches. Successive 

 branching in this way finally produces a system which ends 

 in innumerable minute rootlets. 



52. With the development of the lateral roots the seedling 

 becomes firmly anchored in the soil. This is a necessary 

 preliminary in many plants to the first steps in the development 

 of the stem. In some cases the conical plumule pushes upward 

 through the soil as the radicle grows downward, without moving 

 the cotyledons. In other cases the cotyledons are forced up 

 through the soil before the plumule has undergone any con- 

 siderable development. This is accomplished by the elongation 

 of that part of the seedling called the hypocotyl which lies 

 between the cotyledons and the first lateral roots. With one 

 end fixed by its root anchorage, the elongating hypocotyl 

 carries the cotyledons upward in the direction of least soil 

 resistance. During this process the seed coats are" stripped off, 

 and, as soon as the cotyledons appear above ground, the 

 plumule is free to continue its development. 



53. The several functions of the cotyledons now become 

 evident. In those cases where they remain in the soil they are 

 either greatly swollen by the reserve food contained in the cotyle- 

 dons themselves, or else they are embedded in a large store of 

 endosperm or perisperm. In either case they nourish the grow- 

 ing embryo from the stored food supply. The plumule of such 

 seeds is a conical shaft, well adapted to bore its way through 

 the ground. 



