These negatively geotropic roots have been little studied. ... Typically the blue-green algae in cycad coralloid roots are found localized in a discrete algal zone, which is enclosed by an outer cortex,absent in normal roots. McLuckie (1922) has theorized that the outer cortex corresponds to a persistent root cap.

Introduction:
Coralloid roots are specialized roots found in Cycas, a genus of gymnosperms. These roots have a symbiotic association with cyanobacteria, which fix atmospheric nitrogen and provide it to the plant. Coralloid roots possess certain unique features that distinguish them from typical roots.

Explanation:
The absent features in coralloid roots of Cycas are root hair and root cap. Let's understand this in detail:

Root hair:
Root hairs are outgrowths of epidermal cells that increase the surface area of the root for absorption of water and minerals. However, coralloid roots do not possess root hairs. This is because these roots primarily function in nitrogen fixation rather than absorption of water and minerals from the soil.

Root cap:
Root cap is a protective structure located at the tip of the root. It consists of several layers of cells that cover and protect the delicate root apical meristem. However, coralloid roots lack a root cap. This is because they do not primarily function in soil penetration, but rather in the establishment of a symbiotic association with cyanobacteria.

Casparian band in endodermis:
The Casparian strip or band is a specialized deposition of suberin (a waxy substance) in the radial and transverse walls of the endodermal cells. It acts as a barrier that controls the movement of water and minerals into the vascular cylinder. In coralloid roots of Cycas, the Casparian band is present in the endodermis. This feature is not absent in coralloid roots.

Conclusion:
In summary, coralloid roots of Cycas lack root hairs and root cap, but they possess a Casparian band in the endodermis. These adaptations are specific to their role in nitrogen fixation rather than nutrient absorption from the soil.