ORGANS OF NUTRITION. 127 



Stem from -which the branches arise. These tap-roots do not, 

 however, commonly descend far into the ground, but their 

 branches become much developed laterally ; in some cases even 

 more so than those of the stem, while in others they are less so, 

 as is especially the case in plants of the Gourd tribe, and com- 

 monly in all succulent plants. 



In their internal structure the root resembles the stem except 

 that it has no pith or medullary sheath, so that the woody part 

 forms the central axis of the root. This absence of pith and 

 medullary sheath is generally the case in herbaceous plants, but 

 there are some trees, as, for instance, the Walnut and Horse- 

 chestnut, where the pith is prolonged downwards for some dis- 

 tance into the root. 



2. The Koot or Monocotyledonous Plants. — In these 

 plants the radicle does not itself become prolonged to form the 

 root, but it generally gives off above its base one or more 

 branches of equal size, which separately pierce the radicular ex- 

 tremity of the embryo, and become the roots (^fig. 742, r); each 

 of these is covered at its base, where it pierces the integuments, 

 witli a cellular sheath which is termed the colecr/tiza (Jig. 742, c6). 

 Such a mode of root-development has been termed by Richard 

 endorhizul. The roots of Monocotyledonous Plants are therefore 

 to be regarded as adventitious or secondary. 



From their mode of development it rarely happens that the 

 plants of this class have tap-roots, but they generally consist of 

 a number of separate parts of nearly equal size {fig. 742), and 

 hence are said to be compound. There are, however, exceptions 

 to this, as for instance in the Dragon-tree (fig. 186), which has 

 a central axis resembling the ordinary tap-root of Dicotyledonous 

 Plants. 



Aerial roots are much more common in Monocotyledonous 

 then in Dicotyledonous Plants. We have already referred to 

 them in the Screw-pine (fig. 178, 2). In many Palms they are 

 developed in great abundance towards the base of tlie stem, 

 by which that portion assumes a conical appearance, which is at 

 once evident by the contrast it presents to the otherwise cylin- 

 di'ical stipe of such trees. In their internal structure the roots 

 of Monocotyledonous Plants resemble those of the stem. 



3. The Root of Acottledonous Plants. — Such plants, as 

 we have seen, have no true seeds containing an embryo, but are 

 propagated by spores, which develope roots indifferently from 

 any part of their surface, and hence have been called by Richard, 

 heterorhizal. Such roots are therefore all adventitious. When 

 the stem has become developed it soon gives origin to other 

 adventitious roots, by which such plants are chiefly supported. 

 Hence aerial roots are very common in Acotyledonous Plants, 

 in which respect they resemble those of Monocotyledons. In 

 Tree-Ferns also, as in many Palms, these roots are so abun- 



