THE ANGIOSPERAIAE : ROOTS 



8ii 



normal type by the specialized development of one or other of its character- 

 istic functions or structures, in a manner suitable to special circumstances. 

 When we speak of such structures as being " modifications," we imply 

 that the simple type of absorbing root in the soil was the earliest type to be 

 evolved and that the specialized forms have been derived from it. 



(a) Storage Roots. 



Among the simplest of these special cases are the storage roots. Normal 

 roots usually store food reserves, but in some cases they are enlarged into 



Fig. 806. — Orchis tuorio. Plant showing storage root tubers. 



root tubers, large, rounded structures with massive parenchyma tissues, 

 packed with reserve materials. Sometimes the whole root is involved, as 

 among soil-living Orchids, such as Orchis morio (Fig. 806) ; sometimes the 

 base of the root thickens, as in the tubers of Dahlia variabilis (Fig. 807) ; 

 sometimes small tubers are formed as branches on normal roots, as 



