62 PART I. THE MORPHOLOGY OF PLANTS. [ 13. 



whether normal or adventitious, is developed, not at the surface, 

 but in the interior of the tissue, that is, endogenously . 



There are exceptions to both these rules. The primary root of some para- 

 sitic plants, such as Orobanche and Cuscuta, has no root-cap, as also the small 

 lateral roots which spring from the larger roots of the Horse-Chestnut. In some 

 cases (e.g. old roots of Azolla caroliniana , Hydrocharis, Pistia Stratiotes) a 

 root-cap is present at first, but eventually disappears, the growth in length of 

 the root being arrested. 



Exogenous development has been observed in the adventitious roots of 

 Cardamine pratensis (roots of adventitious buds developed on leaves) ; of 

 Neottia Nidus Avis ; of Nasturtium officinale and silvestre ; of Ruppia rostellata 

 (embryo) ; Lycopodium, Phylloglossum. 



The root which is developed at the opposite pole of the embryo to 

 the shoot, is termed the primary root (see p. 15). When the primary 

 root persists and continues its growth (as in Oak, Radish, Bean, 

 etc.), it is termed a iap-root. In many cases (generally in Monocoty- 

 ledons) the growth of the primary root is limited, so that it attains 

 but feeble development. In other cases {e.g. Orchids, Phyllo- 

 glossum, Selaginella) no primary root is developed, all the roots 

 being adventitious. The symmetry of the root is most commonly 

 radial. In some cases, however, the root is isobilateral, as is shown 

 as well by its internal structure as by the development of two 

 opposite longitudinal rows of lateral roots. In other cases (e.g. 

 attached aerial roots of epiphytic Orchids ; roots of Podostemaceoe) 

 its symmetry is more or less distinctly dorsiventral. 



Boots branch either dichotomously (e.g. Isoetes), or laterally 

 (see p. 33). In lateral branching the secondary roots are developed 

 in acropetal succession on the primary root, and so on. The 

 growing-points of the lateral roots are derived from that of the 

 parent root (see p. 21) ; but, since the endogenous lateral roots 

 take some time in reaching the surface, they are only perceptible 

 externally at some distance from the apex of the parent root. The 

 terminal apical portion of the parent root consequently bears no 

 lateral roots. On anatomical grounds (see Part II.) the secondary 

 roots are arranged in longitudinal rows on the primary roots ; an 

 arrangement which also obtains in the branches of the secondary 

 roots, and of higher orders. 



All roots which are not developed as branches of the primary 

 root, that is, which are developed from other members, as well as 

 all branches which are not developed in acropetal succession on 

 the primary, secondary, or higher order of branches, are said to be 

 adventitious (see p. 21). 



