INTRODUCTION 489 



stems and leaves. Certain liverworts exhibit all gradations between thalli 

 and leafy stems. Even in the seed plants, where differentiation into 

 roots, stems, and leaves seems relatively fixed, there are cases where the 

 exact nature of certain organs is subject to dispute (as in the rhizophores 

 of Selaginella, the plant body of Utricularia, and the spines of cacti). 



The chief characteristics of roots, leaves, and stems. — The root 

 generally is a descending, irregularly branching axis, while the stem gen- 

 erally is an ascending, regularly branching axis, possessing nodes and 

 internodes, and bearing leaves as lateral members and reproductive 

 shoots as lateral or terminal members; branch shoots commonly arise 

 in the leaf axils. Branch roots arise from within (endogenously), con- 

 trasting with branch shoots which arise at the exterior (exogenously) . 

 Young roots possess a root cap which ensheathes the growing tip, and 

 strands of xylem and phloem alternate in the same circumference (p. 683) ; 

 the central region is occupied by conductive tissues, and the epidermis 

 is ephemeral. Young stems contrast with young roots in that the xylem 

 and phloem form continuous or interrupted cylinders about the pith, the 

 phloem commonly being outermost; the epidermis is much less ephem- 

 eral than in roots. Stems and roots commonly are radially symmetrical, 

 possessing an infinite number of vertical planes of symmetry, while 

 leaves are dorsiventrally symmetrical, possessing a single plane of sym- 

 metry. 



The preceding distinctions between roots, stems, and leaves are 

 general but not universal. For example, some roots are exogenous in 

 origin and some stems endogenous. Not all roots have root caps. Some 

 roots bear buds which a e able to develop into leafy shoots; shoots 

 borne on roots are not necessarily subtended by leaves. Old roots and 

 stems may be indistinguishable in structure; many stems and roots are 

 without radial symmetry, and sometimes there is but a single plane of 

 symmetry, as in most leaves; some leaves, however, are all but radially 

 symmetrical (p. 629). These and similar exceptions indicate sufficiently 

 the intergradations between these organs. Indeed, it is doubtful if a 

 single characterization can be made that always holds; occasionally it is 

 only by a majority test of its characters that the nature of an organ 

 may be determined. Sometimes one or more of the plant parts may be 

 missing, as the roots in Salvinia, and as both leaves and roots in Wolffia; 

 many rosette plants, as the dandelion, are practically stemless. Thus, 

 such terms as roots, leaves, stems, and reproductive organs are con- 

 venient words rather than fundamental categories. Here, as in every 



