Root Structure 29 



cotyledon is freed from the integuments, and is raised up 

 through the soil. 



When the cotyledons remain beneath the soil they are said 

 to be hypogeal, when they rise above epigeal. 



There is also a great difference between dicotyledons and 

 monocotyledons with regard to the formation of the young 

 root. In dicotyledons the radicle is directly prolonged, and 

 no branching takes place until after the young root has left the 

 seed (fig. 48), so that the first root is a direct prolongation of 

 the radicle, and is known as a normal root. • Such a mode of 

 root development is called exorhizal. In monocotyledons, on 

 the other hand, there is a branching within the seed (fig. 47), 

 and the radicle is not directly produced. Such roots are known 

 as adventitious, and the growth is said to be endorhizal. 



The term adventitious is applied to all roots which are not 

 developed by the direct growth of the radicle ; hence in dico- 

 tyledons adventitious roots may be developed later on from 

 various parts of the plants (as from stem, as seen in the Ivy, 

 &c.), so that it is only the first-formed roots which are always 

 normal in this group. 



ROOT STRUCTITRE. 



Even whilst the radicle is within the seed a change of shape 

 and condition has come over some of the cells, which, however, 

 is more apparent after the ydung root has commenced to grow 

 (fig. 49). The outer layer of cells becomes more or less flattened, 

 forming a complete protective coat known as the epidermis. 

 At the extremity the cells are thickened in such a manner as to 

 produce what is known as the root-cap or pileorhiza, which is 

 intended to protect the growing extremity of the root during 

 its prolongation. Just behind the root-cap there is a mass of 

 meristem tissue, which forms the growing point of the root. 

 This by the constant subdivision of its cells produces on its 

 outer sides additions to the root-cap, thus making up for 

 portions that have become worn off by the growth of the root, 

 whilst on its inner side it forms fresh tissue, thus causing the 

 root to increase in length. Roots, then, increase in length 

 only by growth near the apex. The upper part of the root is 



