44 PART I. MORPHOLOGY. [ 13 



lapping is by one margin only, in which case the prefoliation is to be 

 semi-equitant. 



13. The Root. It must be clearly apprehended that a sub- 

 terranean member is not necessarily a root ; nor can a member be 

 termed a root because it is found to absorb water and salts in 

 solution, for in rootless plants this function may be discharged by 

 shoots, or leaves, or hairs; nor can a member be termed a root 

 because it serves as an organ or attachment to the substratum, for 

 such organs may be emergences (see p. 48) ; only such members 

 can be regarded as roots which bear neither leaves nor true 

 reproductive organs. 



The root is sometimes wanting in plants where it might be 

 expected to be present, in plants, that is, of which the body is not 

 a thallus (e.g. Salvinia, Psilotum, Utricularia, Epipogum, Coral- 

 lorhiza). 



There are certain peculiarities connected with the structure and 

 development of the root which contribute to its morphological 

 distinction. As a rule, the growing-point of the root is not ex- 

 posed, like that of stems or leaves, but is covered by a structure 

 termed the root-cap. As a rule also, the growing-point of the root, 

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

 but in the interior of the tissue, that is, endogenously (p. 9). 



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

 parasitic 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 oflicinale and nilvestre ; of Ruppia rostdlata 

 (embryo); Lycopodium, Phylloglossum. 



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

 to the primary shoot, is termed the primary root. When the 

 primary root persists and continues its growth (as in Oak, Rad- 

 ish, Bean, etc.), it is termed a tap-root. In many cases (generally 

 in Monocotyledons) the growth of the primary root is limited, so 

 that it attains but feeble development. In other cases (e.g. 

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

 being adventitious. The symmetry of the root is most com- 



