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The main growing points in plants are the shoot and root meristems.

Illustration of a plant showing both roots and shoots, with close up photos showing the meristems.

A flowering plant's apical meristem or shoot tip is described by the tunica-corpus theory of meristem organization, where cell layers or tunica cover the body or corpus of the stem.

Cross section photo of an shoot tip, pointing out the apical meristem, leaf primordia, and lateral buds.

Photomicrograph of a shoot tip showing the apical meristem between two developing leaves.

Satina and Blakeslee (1941) described the tunica of the dicot shoot meristem as having distinct layers.

They called the tunica layers

L-I, L-II, and L-III.

Cross section photo of a dicot shoot meristem, pointing out the corpus and tunica. Cross section photo of a dicot shoot meristem, pointing out the tunica layers L-I, L-II, and L-III

Close up illsutration of a dicot shoot meristem, pointing out layers L-I, L-II, and L-III. It indicates that the upper layers L-I and L-II are part of the two-layered tunica, while the lowest layer L-III is part of the corpus.

Gymnosperm meristems do not have layers, but have zones.

A few apical initials are responsible for new stems and leaves.

An illustration of a gymnosperm meristem pointing out the surface layer cells, apical initials, subapical initials, central mother cells, peripheral zone, and rib meristem.

The root meristem lies under a protective root cap.

Cross section photo of a root meristem pointing out the vascular cylinder, cortex, meristem, and root cap.