ORGANS OF NUTRITION. 121 



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 its internal structure the root resembles the stem except 

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

 . forms a central axis. This absence of pith and medullary sheath 

 '^ is general in herbaceous plants, but there are some trees, as, for 

 instance, the Walnut and Horse-chestnut, where the pith is pro- 

 longed downwards for some distance into the root. 



2. The Eoot of Monocotyledoxous 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 extremity of 

 the embryo, and become the roots (Jiff. 22b, r); each of these roots 

 is surrounded at its base, where it pierces the integuments, with a 



* kind of cellular collar, termed the coleorhiza {fig. 225, co). Such 

 a mode of root-development has been termed endorhizal. The 

 - roots of Monocotyledonous Plants are therefore to be regarded 

 * as adventitmis or secondary. 



From their mode of development it rarely happens that the 

 plants of this class have tap-roots, but they have instead a 

 variable number of roots of nearly equal size {fig. 225, r), which 

 are accordingly termed comjyound. There are, however, excep- 

 tions to this, as for instance in the Dragon-tree {fig. 178), which 

 has an axis resembling the ordinary tap-root of Dicotyledonous 

 Plants. 



■ Aerial roots are much more common in Monocotyledonous than 



in Dicotyledonous Plants. We have already referred to them 

 in the Screw-pine {fig. 170, 2), and other plants. In many 

 Palms they are developed in great abundance towards the base 

 of the stem, by which this portion assumes a conical ap- 

 pearance, which is at once evident by the contrast it presents to 

 the otherwise cylindrical stem of such trees. In its internal 

 structure the root of a Monocotyledon corresponds to that of 

 the stem in the same class of plants. 



3. The Koot of Acotyledonotjs Plants. — Such plants, as 

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



propagated by spores, which develop roots indifferently from any 

 part of their surface, and hence have been called heterorhizal. 

 Such roots are therefore all adventitious ; and resemble those 

 of monocotyledonous plants in being compound. When the 

 stem has become developed it soon gives origin to other ad- 

 ventitious roots, by which such plants are chiefly supported. 

 Hence aerial roots are very common in Aeotyledons, as in 3Iono- 

 cotyledons. In Tree-Ferns also, as in many Palms, these roots 



