For boundary questions involving a long subject, identify the subject and its main verb before evaluating punctuation. Temporarily remove descriptive phrases and dependent clauses; if a nonessential phrase interrupts the core subject-verb structure, make sure punctuation marks both its beginning and its end.
Hints
- Hint 1
Find the main subject
In the clause after “have found that,” identify what performs the action described near the end of the sentence.
- Hint 2
Remove extra information
Temporarily remove the words from “a feature” through “greater depths.” Check whether the remaining words form a complete clause.
- Hint 3
Examine the interruption
The phrase beginning with “a feature” renames the root layer and is not essential to the sentence’s core meaning. Consider how both boundaries of such an interruption should be marked.
Step-by-step
- Step 1
Identify the core clause
Within the clause introduced by “that,” the core structure is “the unusually dense layer ... helps the crop capture water.” The verbs “develops” and “focus” belong to modifying clauses, so neither is the main verb for “layer.”
- Step 2
Set off the appositive
The phrase beginning with “a feature” renames and provides extra information about the layer of roots. Because a comma introduces this nonessential appositive after “surface,” another comma is needed after “depths” to close it.
- Step 3
Complete the sentence
After the appositive is closed, “helps” correctly completes the clause by linking the singular subject “layer” to what it enables the crop to do.