What is the name of the root layer that is only one cell thick, forming a barrier for incoming substances?

The endodermis is the name of the root layer that is only one cell thick, serving as a critical barrier for substances entering the vascular system of a plant. This layer is an important component of the root structure as it regulates the flow of water and nutrients from the soil into the plant. The cells of the endodermis are often tightly packed and may contain a substance called suberin, which forms a Casparian strip. This strip helps ensure that only select substances can pass through the endodermis, effectively controlling what gets absorbed and providing a protective layer against harmful compounds in the soil.

Different parts of the root system serve distinct functions. The vascular cylinder is primarily involved in the transport of water, nutrients, and food. The pericycle contributes to the formation of lateral roots and can also play a role in secondary growth. The cortex is mainly for storage and transport of materials but does not primarily serve as a selective barrier, thus differentiating it from the endodermis' primary function.