ong>Answer:

Roots are one of the primary vegetative organs of extant vascular plants; the others are shoots, leaves and hairs [1,2]. Roots are defined as axial organs of the free-living sporophyte that develop from root apical meristems with root CAPS [1] (the term root will be used explicitly to refer to rooting structures that conform to this definition -see Box 1 for Glossary). ...

... Roots are one of the primary vegetative organs of extant vascular plants; the others are shoots, leaves and hairs [1,2]. Roots are defined as axial organs of the free-living sporophyte that develop from root apical meristems with root caps [1] (the term root will be used explicitly to refer to rooting structures that conform to this definition -see Box 1 for Glossary). There is a suite of characters that together make roots readily IDENTIFIABLE from the other axial vegetative organ of extant vascular plants, the shoot. ...

... The origin of roots with caps must, therefore, have post-dated the divergence of A. mackiei from the LINEAGE including extant MEMBERS of the lycophytes (Figure 1) [10]. Roots developing root caps, the sole defining feature of roots of all extant vascular plants [1], must have evolved after the evolution of the rooting axis and, therefore, have been a relatively late innovation in the lycophyte lineage. As both extant lycophytes and euphyllophytes develop roots with caps today, this INDICATES that roots must have evolved inde- pendently in the two lineages (Figure 1). ...