GENETIC RELATION OE AXIS AND ROOT 219 



X 



evidence from plants of the present day of the existence of a primitive, 

 permanently free-living, but rootless state of the sporophyte. 



The root in the fully-developed state is broadly different from the 

 axis : its endogenous origin, its root-cap, and the radial arrangement of its 

 vascular system are its most distinctive features, in addition to the absence 

 of appendicular organs, other than root-hairs, or lateral roots. Its full 

 character depends upon the collective existence of those features ; for some 

 of them are inconstant, and all of them may occasionally be matched by 

 axes : 1 thus the two parts are not absolutely distinct in character. 

 Sometimes, indeed, it may be found that roots grow on directly into normal 

 leafy shoots, as in certain Ferns, Aroids, and Orchids, etc. : ' 2 the con- 

 verse, however, has not yet been shown to occur. 



The resolution of the problem what genetic relation, if any, subsisted 

 between axis and root will naturally be looked for in such plants as show 

 the least degree of differentiation of those parts. As such the living 

 Lycopods are pre-eminent, while their fossil relatives also show features 

 of importance for comparison. Like axes, the first roots may be exogenous, 

 as in certain Lycopod embryos, and in Phylloglossum : in the Lycopods 

 the roots show apical dichotomy as do their stems also, while the exarch 

 xylem and general disposition of the vascular tissues of the Lycopod stem 

 are points of similarity to root- structures which are not equalled in other 

 Vascular Plants. Finally, the Selaginellas present features of further 

 interest in their so-called " rhizophores," parts which occur in many, but 

 not in all species : they are exogenous in order, and capless : they branch 

 dichotomously, and upon them the roots with root-cap arise endogenously. 

 In structure they are usually like roots, but in some cases the rhizophore 

 has a structure resembling that of an axis : for instance in 6". Kraussiana 

 the protoxylem is central, and the whole arrangement very like that of the 

 stem in S. spinosa? Further, the rhizophores may be readily converted 

 in some species into leafy shoots, by suitable cultivation. Thus the 

 rhizophores do not show the full characters of roots or of axes, and the 

 question has long been debated whether or not they are truly of root- 

 nature. Some prefer to distinguish them by a special name, as "rhizo- 

 phores " : others describe , them merely as the aerial region of the root. 



1 Exogenous roots are seen in Phylloglossum, and in the embryos of some species of 

 Lycopodiitin, as well as in some Phanerogams. Capless roots are known in Aesctilus, and 

 in some few others (Goebel, Organography, vol. ii., quote from Engl. ed., p. 267). On 

 the other hand, a protective cap has been observed on the apex of the axis in embryos 

 of Araucaria, and Cephalotaxus by Strasburger (Angiosp. und Gymnosp., Plates xix. to 

 xxi. ) : endogenous shoots are not common, but they occur occasionally, as in the flower- 

 buds of Pilostylcs, as well as on the emergence of shoot-buds adventitiously from roots 

 (Goebel, I.e., pp. 226, etc.). A radial disposition of the vascular tissue, i.e. with exarch 

 xylem, is characteristic of the axes of Lycopods, and of some others of the early types 

 of Pteridophytes. 



"Goebel, Organography, vol. ii., p. 226. 



3 Harvey Gibson, Ann. of Bot., 1894, PI. x., Fig. 39. Also 1902, PI. xx., Fig. 17. 



