Fig. 2. Diagram of a hypothetical leaf in cross-sectional view illustrating all possible types of sector boundaries. Upper and lower epidermises are demarcated by closely spaced horizontal lines, and are separated by multiple mesophyll layers. Shaded areas represent wild-type tissue; unshaded areas represent w14-marked tan mutant tissue. Wild-type epidermal cells may overlie wild-type mesophyll (A) or mutant mesophyll (B). Mutant epidermal cells may overlie mutant mesophyll (C) or wild-type mesophyll (D). Transverse sector boundaries between the epidermis and mesophyll occur when wild-type epidermal cells overlie mutant mesophyll (e.g., area B) or when mutant epidermal cells overlie wild-type mesophyll (e.g., area D). Lateral sector boundaries in the epidermis are represented by numbered arrows. A lateral boundary in the epidermis may coincide directly with a lateral boundary in the mesophyll (arrow 2). It may also occur over either wild-type (arrow 1) or mutant (arrow 3) mesophyll.