222 A FREE-LIVING SPOROPHYTE 
and dichotomous branching; and these stem-lke characters are actually 
exemplified in the roots of living Lycopods; but in most cases the 
exogenous origin and dichotomous branching gave way to endogenous 
origin and monopodial branching, both of which are more suitable for 
parts which have to make their way through the soil. The exarch xylem 
was, however, maintained; and, being biologically convenient in absorptive 
organs, it became a characteristic feature of the root for Vascular Plants 
at large. Many of the primitive types of Vascular Plants had exarch 
xylem in the stem; and if in the same way their primitive roots resembled 
their stems structurally, they 
also would have exarch xylem. 
On this hypothesis the roots 
would appear to have retained 
a structural character which 
was represented in the early 
structure of stems. In_ this 
way the origin of roots may 
be presented to the mind 
without their being held to 
have been actually the result 
of transformation of a leafy 
shoot itself, of which there is 
no evidence from abortive 
appendages. But as a matter 
of fact, there is no certain 
knowledge how the root 
originated. 
In most embryo — sporo- 
Fic. 114. phytes of the present day a 
Pleuromoia Sternbergii. Swollen hase of stem with root. Yoot-development is initiated 
sears, and showing part of the aerial stem, with the epidermis a ° 3 
and leaf-scars on the right, and on the left the sub-epidermal before the need for it arises, 
sculpture, “(After Bischof from Englerand Prantl.) Two-thirds that is while the embryo e 
still entirely dependent upon 
the parent gametophyte. But it cannot be assumed that this was always 
the case: indeed, it is thought by some that there is an inherent 
probability that some intermediate condition may have preceded the 
initial formation of the root in descent. Among the embryos of some 
of the early types of Vascular Plants a condition has been found which 
has been held to be primitive, and to illustrate how the transition to 
a free-living condition of the sporophyte may have been effected: it is 
seen in certain species of Lycopodium, L. cernuum, and L. tnundatum, 
in which the prothallus is green and subaerial, thereby suggesting a rela- 
tively primitive condition as compared with other types of the genus. In 
these the upper half of the embryo, owing to the rapid extension of the 
massive foot, is soon extruded laterally from the prothallus (Fig. 115). 
