Variation in the number of cells arranged vertically cannot be large because light can penetrate only so many layers of cells. On the other hand, the number of cells arranged horizontally in the leaf continues to 17) __________ as leaves expand, regardless of the size of the individual cells.
The new allometric equations were said to be 18) __________toward understanding the design of leaves on a cellular basis, but there is still much to learn before some 19) __________ might be imitated synthetically. In 20) __________, the group will study species that are very closely related in an effort to uncover any evolutionary relationships between leaf design and function.