122 



ORGANOGRAPHY. 



pith or medullaiy sheath. 4th, It has no true epidermis Avith 

 stomata, but in place of this an integument composed of cells 

 without stomata, to which the name of epihlema has been given. 

 5th, It has no leaves, or scales which are modified leaves. 6th, 

 It has no regular buds, and has consequently, no provision for a 

 regular ramification. 



2. Adventitious or Secondary Root. — This name is ap- 

 phed to all roots which are not produced by the direct elongation 

 of the radicle of the embryo; because these, 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 circumstances. The 

 branches of a true root are of this nature; as are also all those 

 produced from the diiferent modifications which stems assume, 

 as the rhizome, the runner, sucker, stolon, corm, luilb, &c. ; 

 those of slips and cuttings of ])lants, &c. ; and tliose of all 

 Monocotylcdonous and Acotyledonous Plants. All the above 

 are examples of adventitious roots which are develoi)ed from 

 parts in contact with the ground, as the stem does not com- 

 monlv produce roots under other conditions. There are, 

 however, various exceptions to this, as is the case with all those 

 roots which are develoi)ed from plants in the air, and which arc 

 accordingly called Aerial Roots. These adventitious or se- 

 condary roots have tlieir origin in the 

 bark, some of the cells of which near 

 to the vascular bundles acquire a 

 special development, aiul separating 

 from those around tliem, grow in the 

 manner already described, and thus 

 break their way tlirough the bark, 

 and become free. 



The true or primary root, from its 

 being formed by a direct elongation 

 of the radicle, generally continues to 

 grow downwards for some time at 

 least, and hence forms a main trunk 

 or axis from which tlie branches 

 are given 'off {fig. 233). Such a 

 root is termed a tap-root, and may 

 be commonly observed in Dicoty- 

 ledonous Plants. On tlie contrary, 

 those of Monocotylcdonous and Aco- 

 „. „„ - ^ ,. , tyledonous Plants, which are adventi- 



/^»or. 233. Lower part of the stem •. , ii r 



and root of t'.c common Stock, tious or sccoudary, arc Usually ot 

 r.The tap-root with its brandies. „earlv equal sizc, and given ott' in 



c riie neck oi point of union , ' ,- ^i i- i /^ "tn^ 



between Btem ani root, r The clustcrs h'om the radiclc (//f/. /42). 

 stem. /,./: Leaves, b, h. Buds. Somc adventitious roots, such as those 

 called aerial, retiuire a more jiarticular notice. 



Fig. 233. 



