Structure of a Root

Figure 33-4 shows a cross section of a root in a region where only primary growth has occurred. All tissues here derive from apical meristem. The inner most, star-shaped tissue is the primary xylem. Between the points of the primary xylem are areas of primary phloem. Immediately outside of these primary vascular tissues is a zone of parenchyma cells, called the pericycle. This is bounded by a layer of suberized cells one cell in thickness and called the endodermis. Peripheral to the endodermis is another zone of parenchyma cells known as the cortex. The cortex is bounded on the outside by the epidermis.

Figure 33-5 shows there to be a region in the root where one may trace a circlet hat will pass alternately through primary xylem and primary phloem. This alternating arrangement of the primary xylem and primary phloem is one of the principal differences between roots and stems.

Figure 33-4 Cross section of a root exhibiting no secondary growth. The position of the cambium lies between the primary tissues. (b) Passage cells lie in the endodermis.

Figure 33-5 Portion of a root exhibiting only primary growth and lying within the endodermis. A circle traced as shown at (a) passes alternately through primary xylem and primary phloem. Only a root exhibiting no secondary growth possesses this configuration.