GENETIC RELATION OF AXIS AND ROOT 219 
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:! 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.:? 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 Phyl/oglossum: 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 Se/agimedlas 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 S. Kraussiana 
the protoxylem is central, and the whole arrangement very like that of the 
stem in .S. sfinosa.2 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 
Lycopodium, as well as in some Phanerogams. Capless roots are known in Aesez/us, 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 Avaucaria, and Cephalotaxus by Strasburger (Angiosp. und CGymmosp., Plates xix. to 
xxi.) : endogenous shoots are not common, but they occur occasionally, as in the flower- 
buds of Pélostyles, as well as on-the emergence of shoot-buds adventitiously from roots 
(Goebel, Z.c., pp. 226, etc.). A radial disposition of the vascular tissue, .e. with exarch 
xylem, is characteristic of the axes of Lycopods, and of some others of the early types 
of Pteridophytes. 
2Goebel, Organography, vol. ii., p. 226. 
’ Harvey Gibson, Aun. of Bot., 1894, Pl. x., Fig. 39. Also 1902, Pl. xx., Fig. 17. 
