APA

Pennel F. W., . (1935). The scrophulariaceae of eastern temperate north america. Philadelphia: Academy of Natural Sciences of Philadelphia.

Chicago

Pennel F W, . 1935. The scrophulariaceae of eastern temperate north america. Philadelphia: Academy of Natural Sciences of Philadelphia.

Harvard

Pennel F. W., . (1935). The scrophulariaceae of eastern temperate north america. Philadelphia: Academy of Natural Sciences of Philadelphia.

MLA

Pennel F W, . The scrophulariaceae of eastern temperate north america. Philadelphia: Academy of Natural Sciences of Philadelphia. 1935.