Origin of Roots

What is a root?

• Branching systems that are frequently underground and grow in a positively gravitropic manner (towards gravity)

• They have endogenous branching, in which the pericycle creates lateral roots inside of the root and then emerge by growing through the cortex and epidermis

• Roots possess a layer of cells called a root cap, which protects the root apical meristem

• Roots tend to be hydrotropic, growing towards sources of water

• They possess an endodermis, which is a layer of thickened cells that regulate the movement of water into the vascular system

What is a rhizome?

• Rhizomes are stems or stem-like structures that creep horizontally along the ground; They grow diagravitropically (perpendicular to gravity)

• They produce adventitious roots that anchor the plant to the ground

• They can also produce leaves that grow upwards from the rhizome (e.g., ferns). Roots never produce leaves.

• Rhizomes branch through exogenous buds. They do not exhibit endogenous branching

• They do not possess root caps, even though some grow underground 

What are rhizoids?

• Rhizoids are more similar to root hairs but are not roots because they lack vascular tissue

• Like roots, they can absorb water and minerals and anchor small plants to the ground

• Most early land plants, including the bryophytes, possess rhizoids.

The earliest evidence of roots

• Silurian zosterophylls

  • The earliest root-like structure comes from Silurian zosterophylls (e.g., Bathurstia)

  • This plant possessed K-type branching, in which an axis dichotomizes two times in close succession 

  • The first dichotomy has an axis that bifurcates pseudomonopodially continuing the progression of the rhizome. 

  • The other axis bifurcates again in an isotomous manner, creating two new axes: a new larger circinate aerial shoot growing upwards and a thinner root axis growing downwards (positively gravitropic).

  • These structures are “rooting organs” but missing some attributes of true roots: lacking a root cap or root hairs (Gensel et al., 2001) and producing new roots exogenously

• Early Devonian clubmosses

  • An evolution of rooting systems also appears in Early Devonian lycopsids (e.g., Asteroxylon)

  • These plants possessed root-bearing axes which bear quasi-roots

  • Root-bearing axes are not roots but grow in a positively gravitropic direction, but they also possess an epidermis and cuticle, occasional stomata, and scale-like enations.

  • Quasi-roots are similar to the roots of living lycopsids (Hetherington et al., 2021) but lack root hairs and root caps. In addition, they are weakly gravitropic, growing in many directions within the soil, but probably did the bulk of the absorption for the plant.

• Given the evidence from both zosterophylls and early clubmosses, it appears that lycophyte roots evolved from the branching of their shoot systems, with one axis evolving into a root-bearing axis, as we see in K-branching (Gensel et al., 2001). 

• From these root-bearing organs evolved roots with features such as root caps, hairs, a pericycle, and an endodermis. 

• Some living lycophytes, such as spikemosses (Selaginella), still possess a root-bearing organ called a rhizophore, from which true roots arise endogenously (Mello et al., 2018).