“Why is lettuce the most loving vegetable?”/“Because it’s all heart.”

A popular Valentine’s Day riddle is:
 
Q. Why is lettuce the most loving vegetable?
A. Because it’s all heart.
 
“Why is lettuce the friendliest vegetable?” was cited in print in 1969 and “Why is lettuce the most loving vegetable?” in 1979.
 
“Why are artichokes so beloved?”/“They’re known for their hearts” is a related riddle.
 
     
Google Books
Ranger Rick’s Nature Magazine
Volumes 3-4
1969
Pg. 14:
Q. Why is lettuce the friendliest vegetable?
A. Because it’s all heart.
—Mark Harris, Age 8, Illinois
 
Google News Archive
3 October 1979, The Maine Paper (Bangor Daily News insert), “Riddles,”  pg. 16, col. 2:
Why is lettuce the most loving vegetable?
Because it’s all heart.
 
Twitter
tfraymond
‏@tfraymond
Valentine joke of the day: Why is lettuce the most loving vegetable?  Because it’s all heart. http://bit.ly/94sa45
8:23 PM - 7 Feb 2010
 
Twitter
Anna Cho Fenley
‏@annachofenley
“Why is lettuce the most loving vegetable?  Because its all heart.” #VDay jokes from clients.  #daymakers
7:40 PM - 13 Feb 2012
 
Twitter
BigBugTel
‏@bigbugtel
Why is lettuce the most loving vegetable?
Because its all heart
#valentinesjokes
12:37 PM - 14 Feb 2013
 
Twitter
JVargas MusicProject
‏@JVMusicProject
Q: Why is lettuce the most loving vegetable?
A: Because it’s all heart.
#ValentinesJokes
11:26 AM - 13 Feb 2014