ORGANS OF NTJTEITION. 



115 



name of epiblema has been given. 5th, It has no leaves, or 

 Jr scales which are modified leaves. 6th, It has no regular buds, 

 I and has consequently no provision for a regular ramification. 

 T"^ 2. Adventitious or Secondary Eoot. — This name is ap- 

 plied to all roots which are not produced by the direct elonga- 

 tion of the radicle of the embryo ; because such roots, instead 

 of proceeding from a definite point as is the case with the true 

 or primary root, are, to a certain extent at least, accidental in 

 their origin, and dependent upon favourable external circum- 

 stances for their development. All branches of a true root, ex- 

 cept those originally produced from its apex, are of this nature, as 

 are also those of the diflferent modifications of stems, such as the 

 rhizome, runner, sucker, stolon, corm, bulb, &;c. ; those of slips 

 and cuttings of plants, &c. ; and those of all Monocotyledonous 

 and Acotyledonous Plants. In some plants also roots are deve- 

 loped from the stems or branches of plants in the air, and are 

 hence called Aerial Eoots. These are also necessarily of an 

 adventitious nature. 



The adventitious roots of Monocotyledonous plants make their 

 first appearance as little more or less conical bodies in the 

 substance of the parenchyma ; these soon break through the tissue 

 which envelopes them, and appear externally, at first as parenchy- 

 matous prolongations, but ultimately they have a similar structure 



Fig. 225. Fig. 226. 



Fig. 225. Germinating embryo of the Oat. r. Rootlets, each with a sheath 



(coleorliiza), co, at its base. c. Cotyledon, g. Youug stem. Pig. 



226. Magnified plants of the L' sser Duckweed (Lemna minor), with 

 the roots covered by a sheath (jiilecnhiza.) 



