“What happened to the plant in math class?”/“It grew square roots.”

A math riddle has been printed on several images:
   
Q: What happened to the plant in math class?
A: It grew square roots.
   
The “square roots” joke has been cited in print since at least 1961.
 
           
Google Books
Arithmeriddles
By Lowry Waring Harding
Columbus, OH: Association for the Study of Educology, Ohio State University
1961
Pg. 80:
Q. What happened to the plant in the classroom arithmetic corner?
A. It multiplied, divided, and grew square roots.
 
Google Books
May 1963, Boys’ Life “Think and Grin,” pg. 82, col. 2:
Jim: Do you know what happened to the plant in the math room?
Jan: No, what?
Jim: It grew square roots. — Douglas Hansen, McKenzie Bridge, Ore.
 
16 December 1966, News Journal (Mansfield, OH), “Fun Time—The Chuckle Box” by A. Leokum, pg. 33, col. 7:
Bob: Do you know what happened to the plant in Math class?
Ted: No, what?
Bob: It grew square roots.
   
18 June 1967, The Sunday Sun (Baltimore, MD), “Sunday Punch,” Magazine, pg. 26, col. 2:
AFTER the first semester the plant in the math teacher’s class grew square roots.
   
Google Books
School Jokes For Kids
By Peter Crumpton
PeteyRF Creative
2015
Pg. ?:
What happened to the plant in math class?
It grew square roots.
 
Reddit—Jokes
What happened to the plant in math class?
submitted February 17, 2015 by itsnotzack
It grew square roots.
   
Reddit—Jokes
What happened to the plant outside the math class window?
submitted January 8, 2016 by thegeneral400
It grew square roots.
   
Twitter
TVOMathify
@TVOMathify
Q: What happened to the tree outside of the math class? A: It grew square roots! #mathjoke
4:00 PM - 17 Feb 2017
 
Reddit—Jokes
What happened to the plant in math class?
submitted July 12, 2018 by MNCPA
It grew square roots.