222 



A FREE-LIVING SPOROPHYTE 



: 



and dichotomous branching ; and these stem-like 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- 

 phytes of the present day a 

 root-development is initiated 

 before the need for it arises, 

 that is while the embryo is 

 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. inundatum, 

 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). 



FIG. 114. 



Pleuromoia Sternbergii. Swollen base of stem with root- 

 scars, and showing part of the aerial stem, with the epidermis 

 and leaf-scars on the right, and on the left the sub-epidermal 

 sculpture. (After Bischof, from Engler and Prantl.) Two-thirds 

 natural size. 



