In most plants, the root plays two fundamental roles: absorption of water and nutrients, and anchorage. Further, in many plants, roots have become specialized for food storage and other functions.
Link to entire set of root images
Tap Root System: Some plants' root systems are organized around a large central root. Tap roots are found both in woody plants and in herbaceous plants.
Link to view of a tap root of a tree
Link to view of tap roots in two herbaceous plants
Fibrous Root System: Other plants' root systems are not organized around a large central root. These root systems consist of a mass of small roots usually arrising from stem tissue (they are adventitious roots). This is the common form of the root systems of grasses.
Link to view of a fibrous roots of a sedge
Other examples of adventitious roots:
Link to view of a prop roots of sorghum
Link to view of clinging roots of poison ivy
Link to those of a cutting of Coleus
The Actively Growing Root Tip: The apical meristem of the root produces new cells. Some derivatives of the apical meristem become part of the root cap and eventually slough off. Others become organized into one of the three primary meristematic tissues (protoderm, ground meristem, procambium).
Link to root tip with primary meristematic tissues indicated
Link to views of grass seedling root
Link to views of longitudinal section ofTriticum (wheat) root tip
Cross Sections of a Herbaceous Dicot Root: Ranunculus (buttercup) roots have a central cylinder of vascular tissue surrounded by a region of ground tissue called the cortex. The cortex is a broad region of cells, the innermost layer of which is called the endodermis. The outermost layer of the vascular tissue (directly inside the endodermis) is called the pericycle.
Link to views of an immature Ranunculus root
Link to views of a mature Ranunculus root
Cross Sections of a Monocot Root: Monocot roots have a central area of parenchyma (pith) surrounded by a layer of vascular tissue with alternating strands of xylem and phloem. These in turn are surrounded by a pericycle (single layered in Zea, multilayered in Smilax). Again, the cortex is a broad region of cells, the innermost layer of which is called the endodermis.
Link to view of Zea (Corn) root cross section
Link to views of a Smilax (greenbriar) root
Origins of Lateral Roots: Lateral roots arise from inner tissues of the root (endogenously), specifically from the pericycle.
Link to views of origins of lateral roots in Salix (willow)
Link to view of longitudinal section of Eichhornia (water hyacinth) showing a series of root primordia arising from the pericycle.