Interpretation of Sum Sq in ANOVA with numeric independent variable

I don't know how to interpret the Sum of Squares in a numeric independent variable.

summary(aov(Petal.Width ~ Petal.Length + Species, iris))
##               Df Sum Sq Mean Sq F value   Pr(>F)    
## Petal.Length   1  80.26   80.26 2487.02  < 2e-16 ***
## Species        2   1.60    0.80   24.77 5.48e-10 ***
## Residuals    146   4.71    0.03                     
## ---
## Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

The sum of squares in Species are clear to me (sum of squared deviations from the group means) but how to interpret them if you have a numeric independent variable like Petal.Length?