470 



On the Structure of Roots. 



many other diversities, and serves as a key to them, or indication 

 of their presence ; and some of these diversities are presented in 

 the anatomy and development of the roots in which we are more 

 immediately interested. Not only do the Dicotyledonous or 

 Monocotyledonous plants differ in the structure of their stems, 

 the characters of their leaves and their flowers, but the first 

 manifestation of independent life, the germination of the seed 

 and development of the embryo, exhibits a peculiaiity in each of 

 these two great classes. In the Dicotyledons the radicle or rudi- 

 mentary root grows directly downward, constituting an inferior 

 prolongation of the stem (fig. 1) ; in the Monocotyledons this 

 rudimentary root is never developed, but lateral rootlets are 

 thrown out at the sides, so that from the first these plants have a 

 tuft of fibrous (fig. 2) or thread-like roots, contrasting with the 



Fis;. 2. 



GeTminating barley: a, seed sprouting; h, more 

 advanced, the rootlets clothed with fibrils. 



Seedling plant of Field Speedwell ( Teronica.) 



more or less branched single tap-root of the Dicotyledons (fig. 3). 

 These substitutes for the tap-root, called by botanists adventitious 



