Plant was pretty instrumental in jump starting urban growth in both cities. Here's the population of both between 1850 and 1930. Plant's first investment in Jax was streetcars in 1879. A year later, he extended his railroad to Jax (basically where the CSX building sits today). In 1883, his railroad reaches Tampa.
Tampa
1850 - 974
1870 - 796
1880 - 720 (Plant's railroad reaches Tampa in late 1883. Vicente Martinez Ybor moves his cigar operations to Tampa from Key West. Cigars shipped to US market by Plant's railroad.)
1890 - 5,532
1900 - 15,839
1910 - 37,782
1920 - 51,608 (Tampa annexes West Tampa, the other cigar manufacturing suburb, in 1925)
1930 - 101,161
Jacksonville
1850 - 1,045
1860 - 2,118
1870 - 6,912 (Plant establishes the Jacksonville Street Railway Company in 1879)
1880 - 7,650 (Plant's railroad is extended from Waycross to Jacksonville in 1881. Jax's populations increases 125% in the years between Plant's railroad arrival and Flagler extending the FEC across the river)
1890 - 17,201 (Flagler extends the FEC across the St. Johns in 1890)
1900 - 28,429
1910 - 57,699
1920 - 91,558
1930 - 129,549