Martha Kramer Cox, 90, a retired teacher of biology and ecology in Baltimore County schools, died of a stroke May 24 at her home in Mitchellville.

Mrs. Cox was born in Beech Grove, Ind., and attended Westhampton College at the University of Richmond from 1931 to 1933. She graduated from the University of Virginia in 1935 with a bachelor's degree in biology. At that time, the University of Virginia did not accept freshman or sophomore women, and allowed upperclasswomen to transfer into only a small number of programs. Her mother had received a degree from a teachers college, however, so Mrs. Cox expected to graduate as well.

She married while in college, then interrupted her career to raise a family. She became an assistant librarian in the Newport News, Va., schools until the late 1950s. The family moved, and she taught biology and ecology in Baltimore County high schools from 1963 until her retirement in 1972. Among her achievements was writing a countywide curriculum for a horticulture course.

She and her husband were members of the Potomac Appalachian Trail Club. The family often rented cabins and went hiking. "She and Daddy stopped along the way and debated the names of plants," said one of her daughters, Martha Cox.

She often attended Camps Farthest Out, a Christian retreat organization with locations across the country. She was a member of Calvary Baptist Church in Towson, Md.

Her husband of 57 years, Dr. George W. Cox Jr., died in 1991. A daughter, Patricia Sue Hardesty, died in 1984.

Survivors include three children, Bonnie Ann Cox of Princeton Junction, N.J., G. William Cox III of Hebron, Conn., and Martha Elizabeth Cox of Waterford; nine grandchildren; and five great-grandchildren.