Concept:
Diagnostic pharmacognostic features help in identifying crude drugs by their microscopic characters.
Step 1: Fennel is an aromatic fruit drug rich in volatile oil.
Step 2: It shows volatile oil cells and schizogenous oil cavities.
Step 3: These structures store volatile oil responsible for aroma and medicinal activity.
Step 4: Ginger, clove, and asafoetida have different diagnostic features.
\[
\boxed{\text{Fennel}}
\]