In O. Henry's short story "The Last Leaf," when does the turning point begin?

O. Henry's poignant story of two young ladies from
disparate parts of the country who live together in artistic harmony finds Johnsy from
California having succumbed to pneumonia while her friend Sue endeavors to keep Johnsy
from succumbing to the disease.  When Johnsy declares that she will die when all the
leaves on the vine outside their window fall off, Sue becomes extremely anxious.  For,
there is a strong wind outside.  Desperate to find something to distract Johnsy from her
morbid thoughts, Sue goes downstairs in the apartment building.  There she contacts Mr.
Behrman who is past sixty and has yet to paint his masterpiece.  Sue convinces Behrman
to come upstairs and pose for her. 

Having told Behrman how
ill Johnsy is, his eyes fill with tears as he talks with Sue.  Upstairs, they "looked at
each other for a moment without speaking." The next morning, Johnsy
awakens and asks Sue to raise the shade: "Put it up; I want to see,"
she orders Johnsy.  This is the turning point, the highest
point of emotional intensity in the story.  Fearfully, Johnsy lifts the green shade,
revealing one yellowed leaf, the "last leaf."  Nevertheless, Johnsy says that the leaf
will fall and she will die when it does.  But, the tenacious leaf remains into the next
day.  So, Johnsy declares,

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 "I've been a bad girl,,,Something has made that
last leaf stay there to show me how wicked I was.  It is a sin to want to
die."

She lives.  But, the
falling action finds that poor Behrman has succumbed to pneumonia now, himself.  It
seems he painted his masterpiece after all.