192 



THE RATE OF GROWTH 



[CH. 



Graded growth of bean-root 



"... I marked in the same manner as the Vine, young Honeysuckle shoots, 

 etc. . . . ; and I found in them all a gradual scale of unequal extensions, those parts 

 extending most which were tenderest," Vegetable Staticks, Exp. cxxiii. 



The lengths attained by the successive zones He very nearly 

 on a smooth curve or gradient; for a certain law, or principle 

 of continuity, connects and governs the growth-rates along the 

 growing axis. This curve has its family likeness to those differential 



10 



2 3 4 5 6 7 8 



Zones 

 Fig. 49. Rate of growth of bean-root, in successive zones 

 of 1 mm. each, beginning at the tip. 



curves which we have already studied, in which rate of growth was 

 plotted against time, as here it is plotted against successive spatial 

 intervals of a growing structure ; and its general features are those 

 of a curve, a skew curve, of error. Had the several growth-rates 

 been transverse to the axis, instead of being longitudinal and 

 parallel to it, they would have given us a leaf-shaped structure, 

 of which our curve would represent the outline on either side; or 

 again, if growth had been symmetrical about the axis, it might have 

 given us a turnip-shaped soHd of revolution. There is always an 

 easy passage from growth to form. 



