What is the title of Darwin's paper on cellular structure?
Darwin had written lots of papers which dealt with fertilization (which involves single cells), as he primarily dealt with the reproduction, continuation and thus evolution of a species, but these did not deal with cell biology specifically.

After a closer look at the complete list of all his publications, I found that he did have two cell-related publishings, specifically about the size of cells, one entitled Bees in Jamaica increase the size and substance of their cells, and the other, Bee-cells in Jamaica not larger than in England, in which he retracts the statement he had made in the previous article.  

Ultimately, to answer your question, it seems as though none of his works dealt with cellular structures.



References


Charles Darwin. "Bees in Jamaica increase the size and substance of their cells". Journal of Horticulture (15 July): 305.
Charles Darwin. "Bee-cells in Jamaica not larger than in England." Journal of Horticulture (22 July): 323.