13.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBERS. 61 



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

 subterranean 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 of attachment to the substratum, for 

 such organs may be emergences (see p. 66) ; only such members 

 can be regarded as roots which bear neither leaves nor true 

 reproductive organs. 



It is not always easy to distinguish at once between root and other members. 

 Thus, in some of the lower simpler forms of plants (e.g. Algse, gametophyets 

 of some Vascular Cryptogams) it is difficult to distinguish between roots and 

 root-hairs, for they are identical in structure, and in neither case do they bear 

 leaves or reproductive organs, But a study of their development affords the 

 distinction. For example, the first stage in the development of the gameto- 

 phyte of Equisetum, and some other Vascular Cryptogams, consists in the 

 division of the spore by a transverse wall into two halves, the epibasal and 

 the hypobasal. The former grows out into a multicellular filament which 

 developes into the thalloid shoot ; the latter grows out, in a diametrically 

 opposite direction, into a delicate unicellular filament, the primary root. It will 

 be observed that, at the time of their first development, root and shoot are here 

 co-ordinate in structure (p. 6). In the course of its growth, the thalloid shoot 

 produces outgrowths from its cells on the ventral surface, which are identical 

 in structure with the primary root. These are not, however, roots, but root-hairs ; 

 for they are not co-ordinate in structure with the shoot at the time of their 

 development. Again, the simple tubular outgrowths on the under surface of 

 the creeping stem of Caulerpa (Fig. 17), are not root-hairs, but roots, for they 

 are co-oidiuate in structure with the stem. Similarly, the distinction between 

 roots and emergences of the nature of haptera and haustoria (see p. 66) is 

 not always immediately obvious. Thus, in its endogenous development, the 

 haustorium of Cuscuta resembles a root, but it differs from a root in that it is 

 developed from the cortex. Finally, subterranean shoots often closely resemble 

 roots, but may be distinguished by the small scaly leaves which they bear. 



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. gametophyte of Mosses : sporophyte of Salvinia, 

 Psilotum, Utricularia, Epipogum, Corallorhiza). 



In the sporophyte of the Vascular Cryptogams and Phanerogams, 

 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, 



