In some cases there may be two or more Tunica layers. However, only the outer layer contributes to the Epidermis. The other two contribute to interior tissues. Their apical organization can appear to be complex when the cytology of their cells, and their organizational patterns are considered. Meristematic cells are supposed to be dividing cells.
Dividing cells are relatively small .
However, the central cells in gymnosperm apices are often very large and they appear to divide rarely. Consequently, the Tunica Corpus model may not be sufficient to describe the architecture of these apices. However, it is sufficient to illustrate the general features of gymnosperm SAMs. It is hard to delimit regions of the SAM based on cell size and staining. The outer Tunica layer is obvious in virtually all SAMs.