Files at WorldConnect give John middle name, "Austin," but I haven't found any records giving him a middle name or even a middle initial.

1.  Missouri Marriage Records, 1805-2002 (index plus scanned images of marriage book pages; online at Ancestry.com; abstracted by Diana Gale Matthiesen):

John STROUP (over age 21) and Jessie HARINE (over age 18), both of Richwoods, Washington Co., MO, obtained a marriage license at Potosi, Washington Co., MO, on 29 Nov 1898.  They were married by Joseph L. Page, J.P., in Richwoods, Washington Co., MO, on 30 Nov 1898.

Samuel L. CLEMENS (æ 34) of Vineland, Jefferson Co., MO, and Jessie STROUP (æ 24 — indexed "Strouf") both of Vineland, Jefferson Co., MO, obtained a marriage license on 19 Aug 1904 in Jefferson Co., MO.

Charles B. MERCER (over age 21) of DeSoto, Jefferson Co., MO, and Jessia A. CLEMENS (over age 18) of Vineland, Jefferson Co., MO, obtained a marriage license at Potosi, Washington Co., MO, on 18 Jul 1913.  They were married by Judge James A. Hen??n on 18 Jul 1913.

2.  1880 Census Every-Name-Index/Images (online at Ancestry.com):  can't find John, not with any certainty.  Francis HORINE (æ 38, b. MO — indexed "Harine") and his wife, Eliza (æ 32, b. IL), are living in Richwoods, Washington Co., MO (p. 95B), with their children.  Charles MERCER (æ 23, b. NY) is living in Thompson, Sullivan Co., NY (p. 320A) with his parents, James D. MERCER (æ 63, b. NY) & Mary (æ 55, b. NY), plus siblings.

3.  1890 Census:  the 1890 Census Population Schedules were destroyed.

4a.  1900 Census Every-Name-Index/Images (online at Ancestry.com, Image #18 of 55):  DeSoto City, Valle Twp., Jefferson Co., MO, Roll T623_867, p. 190B, SN 9, SD 10, ED 61, enumerated 14 Jun 1900, official enumeration date 1 Jun 1900 (extracted by Diana Gale Matthiesen):¤•

1900:  for an explanation of the column headings, please see What the Numbers in the Federal Census Mean (missing columns contained no data).
1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	19	20	22	23	24	25	27
160	164	Stroup John	Head	W	M	Jan 1876	24	M	2			MO	MO	MO	Day Laborer	2	N	N	Y	R	H
		______ Jessie	Wife	W	F	Mar 1880	20	M	2	1	1	MO	MO	MO	Keeping House		Y	Y	Y
		______ Bessie	Dau	W	F	Nov 1899	6/12	S				MO	MO	MO
		______ Rosa	Step-Dau	W	F	Jul 1895	4	S				MO	MO	MO

4a.  1900 Census Every-Name-Index/Images (online at Ancestry.com):  Charles MERCER (æ 42, b. Oct 1857, NY) and his wife, Viania, plus six children; they are listed next to James & Mary BRADFORD, whose son, Thomas BRADFORD, will later marry our subject's adopted daughter, Rosa.

5.  1910 Census Every-Name-Index/Images (online at Ancestry.com):  John is apparentlly deceased.  Can't find Samuel CLEMENS, Jessie, Bessie, or Austin, and I really looked, but I suspect a portion of the 1910 Jefferson County census is missing because numerous other families who should be there are also missing.  Rosa is married and living with her husband, Thomas BRADFORD.  Frank HORINE (æ 70, b. MO) and his wife, Liza A. (æ 61, b. MO), are living in Richwoods Twp., Washington Co., MO (p. 264B), with their children — Jessie, is not with them.  Charles B. MERCER (æ 56, b. NY) is a widower living in Valle Twp., Jefferson Co., MO (p. 256B), two households from his married sister, Jane (MERCER) BRADFORD.

6.  1920 Census Every-Name-Index/Images (online at Ancestry.com):  Jessie A. MERCER (æ 41, b. IL/MO/MO) is living in DeSoto, Valle Twp., Jefferson Co., MO (p. 225A), with her husband, Charles B. MERCER (æ 63, b. NY/NY/NY), plus three children, surnamed MERCER: Melvin M. (5), Edward W. (2y4m), and Charlotte B. (1y6m). They are listed next to Jessie's married (adopted) daughter, Rosie (STROUP) BRADFORD.   Bessie is living with her husband, Claude DICKERSON; Austin is boarding with another family. 

7.  1930 Census Every-Name-Index/Images (online at Ancestry.com):  Jesse A. MERCER (æ 59, b. MO/MO/MO) is living in Valle Twp., Jefferson Co., MO (p. 262A), with her husband, Charles B. MERCER (æ 75, b. NY/NY/NY), plus three children surnamed MERCER: Edward W. (13), Charlotte B. (11), Ernest D. (10), plus a grandchild, also surnamed MERCER.  Rosa and Austin are living with their spouses; Bessie is deceased.

8.  SSDI:  Social Security Death Index (online at RootsWeb.com):

Name	Birth	Death	Last Residence	Last Benefit	SSN	Issued	Post-em
Jessie A MERCER	11 Mar 1880	15 Jul 1966	63126 (Saint Louis, Saint Louis, MO)	(none specified)	498-10-9802	MO	Yes

Is this Jessie A. (HORINE) MERCER?  She is the only MERCER at that zip code.

9.  WorldConnect / Ancestry World Trees (online at RootsWeb.com/Ancestry.com).

10.  One World Tree (online at Ancestry.com).  Gives Jessie's birthdate as 3 Mar 1880 and deathyear as 1954.

11.  Messages posted to the Stroup Family Genealogy Forum (online at GenForum).