a. Medullary parenchyma.
b. Woody tissue.
d. Cortical parenchyma.
This image is from Triticum in Sayre's Manual of Organic Materia Medica and Pharmacognosy, 1917.
Henriette's herbal is one of the oldest and largest herbal medicine sites on the net. It's been online since 1995, and is run by Henriette Kress, a herbalist in Helsinki, Finland.
http://www.henriettes-herb.com - Copyright 1995–2022 Henriette Kress.