A tracheid of oak.
Harry Marshall Ward (1854–1906)
1892
Marshall Ward H. The Oak: A Popular Introduction to Forest-Botany. – London: Kegan Paul, Trench, Trübner and Co. Ltd., 1892. – <a rel="nofollow" class="external text" href="https://www.biodiversitylibrary.org/item/59643#page/77/mode/1up">Figure 16</a>