1. A microscopic image fusion method based on region growing, comprising steps of: (1) setting {I S1 (x,y)} to one microscopic image collected under an optical microscope, and setting {I S2 (x,y)} to another microscopic image collected under the optical microscope, wherein 0≦x≦W−1, 0≦y≦H−1, W denotes a width of an image, H denotes a height of the image, I S1 (x,y) denotes a pixel value of a pixel point whose coordinate position is (x,y) in the {I S1 (x,y)}, and I S2 (x,y) denotes a pixel value of a pixel point whose coordinate position is (x,y) in the {I S2 (x,y)}; (2) dividing the {I S1 (x,y)} into W × H n × n non-overlapping first image blocks, each of which having a size of n×n, recording a first image block with a coordinate position of (i,j) in the {I S1 (x,y)} as I B1 — n (i,j), dividing the {I S2 (x,y)} into W × H n × n non-overlapping second image blocks, each of which having a size of n×n, and recording a second image block with a coordinate position of (i,j) in the {I S2 (x,y)} as I B2 — n (i,j), wherein 0 ≤ i ≤ W n - 1 , 0 ≤ j ≤ H n - 1 , and n is 2-power; (3) evaluating a definition of every first image block in the {I S1 (x,y)} for obtaining a definition characteristic value corresponding to every first image block in the {I S1 (x,y)}, and recording a corresponding definition characteristic value thereof as F v1 — n (i,j) for the first image block I B1 — n (i,j) with the coordinate position of (i,j) in the {I S1 (x,y)}; evaluating a definition of every second image block in the {I S2 (x,y)} for obtaining a definition characteristic value corresponding to every second image block in the {I S2 (x,y)}, and recording a corresponding definition characteristic value thereof as F v2 — n (i,j) for the second image block I B2 — n (i,j) with the coordinate position of (i,j) in the {I S2 (x,y)}; (4) determining a first decision threshold of the definition characteristic value corresponding to every first image block in the {I S1 (x,y)}, recording the first decision threshold as T n — 1 ; and then judging whether every first image block in the {I S1 (x,y)} is a fuzzy seed block according to the definition characteristic value corresponding to every first image block in the {I S1 (x,y)} and the first decision threshold T n — 1 , wherein for the first image block I B1 — n (i,j) with the coordinate position of (i,j) in the {I S1 (x,y)}, judge whether the definition characteristic value F v1 — n (i,j) corresponding to the I B1 — n (i,j) is smaller than the first decision threshold T n — 1 , if it's OK, then judge the I B1 — n (i,j) as the fuzzy seed block and set a mark M B1 — n (i,j) of every pixel point in the I B1 — n (i,j) to 1, if not, judge the I B1 — n (i,j) as the non-fuzzy seed block and set the mark M B1 — n (i,j) of every pixel point in the I B1 — n (i,j) to 0; and then according to the mark M B1 — n (i,j) of every pixel point in every first image block in the {I S1 (x,y)}, calculating a definition tagged image corresponding to the {I S1 (x,y)} and recording the definition tagged image as {I D1 — n (x,y)}, wherein I D1 — n (x,y)=M B1 — n (int(x/n),int(y/n)), I D1 — n (x,y) denotes a mark of a pixel point with a coordinate position of (x,y) in the {I D1 — n (x,y)}, and int( ) is a rounding operator; determining a second decision threshold of the definition characteristic value corresponding to every second image block in the {I S2 (x,y)}, recording the second decision threshold as T n — 2 ; and then judging whether every second image block in the {I S2 (x,y)} is a fuzzy seed block according to the definition characteristic value corresponding to every second image block in the {I S2 (x,y)} and the second decision threshold T n — 2 , wherein for the second image block I B2 — n (i,j) with the coordinate position of (i,j) in the {I S2 (x,y)}, judge whether the definition characteristic value F v2 — n (i,j) corresponding to the I B2 — n (i,j) is smaller than the second decision threshold T n — 2 , if it's OK, judge the I B2 — n (i,j) as the fuzzy seed block and set a mark M B2 — n (i,j) of every pixel point in the I B2 — n (i,j) to 1, if not, judge the I B2 — n (i,j) as the non-fuzzy seed block and set the mark M B2 — n (i,j) Of every pixel point in the I B2 — n (i,j) to 0; and then according to the mark M B2 — n (i,j) Of every pixel point in every second image block in the {I S2 (x,y)}, calculating a definition tagged image corresponding to the {I S2 (x,y)} and recording the definition tagged image as {I D2 — n (x,y)}, wherein I D2 — n (x,y)=M B2 — n (int(x/n),int(y/n)), I D2 — n (x,y) denotes a mark of a pixel point with a coordinate position of (x,y) in the {I D2 — n (x,y)}, and int( ) is a rounding operator; (5) setting n=128, n=64, n=32, n=16 and n=8, respectively repeating steps (3) to (4) for obtaining a definition tagged image {I D1 — 128 (x,y)} corresponding to the {I S1 (x,y)} and a definition tagged image {I D2 — 128 (x,y)} corresponding to the {I S2 (x,y)} while n=128; a definition tagged image {I D1 — 64 (x,y)} corresponding to the {I S1 (x,y)} and a definition tagged image {I D2 — 64 (x,y)} corresponding to the {I S2 (x,y)} while n=64; a definition tagged image {I D1 — 32 (x,y)} corresponding to the {I S1 (x,y)} and a definition tagged image {I D2 — 32 (x,y)} corresponding to the {I S2 (x,y)} while n=32; a definition tagged image {I D1 — 16 (x,y)} corresponding to the {I S1 (x,y)} and a definition tagged image {I D2 — 16 (x,y)} corresponding to the {I S2 (x,y)} while n=16; a definition tagged image {I D1 — 8 (x,y)} corresponding to the {I S1 (x,y)} and a definition tagged image {I D2 — 8 (x,y)} corresponding to the {I S2 (x,y)} while n=8; (6) based on the pixel points with the mark of 1 in the {I D1 — 128 (x,y)}, {I D1 — 64 (x,y)}, {I D1 — 32 (x,y)}, {I D1 — 16 (x,y)} and {I D1 — 8 (x,y)}, making a fuzzy region growing for obtaining a region growing image corresponding to the {I S1 (x,y)} and recording the region growing image as {I G1 (x,y)}, wherein I G1 (x,y) denotes a mark of a pixel point with a coordinate position of (x,y) in the {I G1 (x,y)}; based on the pixel points with the mark of 1 in the {I D2 — 128 (x,y)}, {I D2 — 64 (x,y)}, {I D2 — 32 (x,y)}, {I D2 — 16 (x,y)} and {I D2 — 8 (x,y)}, making a fuzzy region growing for obtaining a region growing image corresponding to the {I S2 (x,y)} and recording the region growing image as I G2 (x,y), wherein I G2 (x,y) denotes a mark of a pixel point with a coordinate position of (x,y) in the {I G2 (x,y)}; and (7) according to the region growing image {I G1 (x,y)} corresponding to the {I S1 (x,y)} and the region growing image {I G2 (x,y)} corresponding to the {I S2 (x,y)}, fusing the {I S1 (x,y)} and the {I S2 (x,y)} for obtaining a final fusion microscopic image and recording the final fusion microscopic image as {I F (x,y)}, wherein I F ( x , y ) = { I S 1 ( x , y ) , if I G 1 ( x , y ) = 0 and I G 2 ( x , y ) = 1 I S 2 ( x , y ) , if I G 1 ( x , y ) = 1 and I G 2 ( x , y ) = 0 C if I G 1 ( x , y ) = I G 2 ( x , y ) , here, I F (x,y) denotes a pixel value of a pixel point with a coordinate position of (x,y) in the {I F (x,y)}, C = { I S 1 ( x , y ) , if C nt 1 ( x , y ) < C nt 2 ( x , y ) I S 2 ( x , y ) , if C nt 1 ( x , y ) > C nt 2 ( x , y ) I S 1 ( x , y ) + I S 2 ( x , y ) 2 , if C nt 1 ( x , y ) = C nt 2 ( x , y ) , C nt1 (x,y) denotes an amount of pixel points with a mark of 1 in an 8 neighbor of a pixel point with a coordinate position of (x,y) in the {I G1 (x,y)}, and C nt2 (x,y) denotes an amount of pixel points with a mark of 1 in an 8 neighbor of a pixel point with a coordinate position of (x,y) in the {I G2 (x,y)}.