I am exremely grateful to Diane Mase who is a direct descendant of Watson Root (an older brother of Orville) for sharing the followng informatin with me:
Orville's parents were Henry Root and Emma Adams. Emma died when Orville was just a young child and we can see that in 1850, when Orville was just 10 years old, his father Henry had remarried to a lady named Betsey. The family was living with who we believe to be Orville's grandparents Ashen and Rachel Gridley at Canton, Hartford County, Connecticut. Later reference to the name Gridley indicates that Henry's second wife Betsey was Betsey Gridley before she married Henry.
Whilst Orville's siblings Sophia (16), Waren (16), Watson (12), Amelia (8), Norris (5), Thomas (3) and Mertin (1). are listed, there was no sign of Orville with them at that time.

Diane has evidence to show that Henry and his first wife Emma Adams had 5 children - Watson, Warren, Orville, Sophia and Amelia. Henry apperas to have had 4 sons with his second wife - Merton, Thomas, Wilbert and Norris.

As Norris was 5 years old in 1850, and we know he was born to Henry's second wife, we can estimate that Henry's first wife Emma died no later than 1844 when Orville would have been just 3-4 years old.

In 1860 we can see the family stil living at Canton, Hartford County, Connecticut.
"Orvia" ROOT (age 20) living with his parents Henry ROOT (49) and [step]mother Betsey ROOT (48); and his siblings Sophia (26), [Charles] Warren (24), Watson (22), Amelia (18) "Harris" (16) Thomas (13), Merton (11), Frederick (9), and Wilford (4). WATSON listed occupation as mechanic.

"Harris" is probably Norris and "Orvia" is Orville.
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Orville enlisted in the Union Army as a Private in D Company, of 13th Connecticut Infantry Regiment during the Civil War, the same regiment as his brother Watson. He was discharged with disabilities on 27 Aug 1862. His pension documents indicate he may well have been paid a pension from 1862 through to 1934.
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Diane also tells me that in the Civil War Pension file for Watson Root, his wife Corinth does mention Orville and his daughter Florence in her deposition. Corinth wrote:
"His father was Henry Root. His mother was Emma Adams, his own mother. His stepmother was Betsy Gridley. They are all dead. He had two own brothers and two own sisters, and four step-brothers. They were [Charles] Warren of Plainville CT., Orville of Unionville CT., who lives with his daughter there, Florence Nibbs; the own sisters were Sophie Case and Amelia Newell, both dead. The step brothers were Merton Root, dead, Thomas, dead, Wilbert dead, and Norris, also dead."

The deposition was taken on February 28, 1916. So, Orville was living in Unionville, CT with Florence at that time.

Orville also gave a deposition on March 1, 1916 in Unionville ("West District"). His name is listed as "Orville Albert Root". [The depositions were taken because there was some question on the part of the federal government as to whether or not Corinth and Watson had ever separated or divorced. If they had, she would not have been entitled to continue to receive his Civil War Pension after he died. She was finally approved.]

So, this is the firm evidence previously missing that Orville was indeed the father of Florence. Orville's wife Ann died in December 1915, so presumably Orville moved in to live with his daughter and son-in-law.
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Orville can be seen at the 1880 US Federal Census living at Southington, Hartford County, Connecticut with his wife Eveline and 4 children.

Orville ROOT Self M Male W 40 CT Carpenter CT CT
Eveline ROOT Wife M Female W 38 CT Keeps House CT CT
Beeman ROOT Son S Male W 15 CT CT CT
Florence ROOT Dau S Female W 13 CT CT CT
Frank ROOT Son S Male W 8 CT CT CT
Eugene ROOT Son S Male W 5 CT CT CT

His daughter went on to marry a man named Charles Evarts in 1891. Charles died on 15 March 1895. Florence then married John Edward Knibbs in June 1897.

Orville's wife Eveline died in February 1894, and Orville then married the sister-in-law of his daughter Florence, Ann Knibbs, herself a widow.
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We can see Orville and Ann in the 1900 census return so clearly they married before that time.
Orville A ROOT Head Apr 1840
Ann ROOT Wife May 1852
Arthur O ROBERTS Son Dec 1887
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We can also see Orville and Annie in the 1910 census returns, living at Hartford, Connecticut, with Orville given as "Orrel":
Orrel Root, Hartford, CT b. abt 1840
Annie Root, Hartford, CT b. abt 1852

Eveline was the daughter of Abel Curtiss Sanford and his wife Hepsa Elizabeth Judd