ii8 THE BIOLOGY OF FLOWERING PLANTS 



be filled by the small leaves of short side shoots. A further 

 complication is seen here, and is of widespread occurrence : 

 the leaves arising on the upper side of the branch remain 

 smaller, and have shorter stalks, than those to the flanks 

 and below. In trees like the maple this anisophylly is 

 solely determined by the relation to light. In some 

 plants it is hereditary, and may go so far that the smaller 

 leaves are reduced almost to scales, as in Procris. Fine 

 examples of anisophylly are the hemlock spruce and the 

 silver fir. The needles are borne spirally, but by torsions 

 and bendings they come to lie in one plane, those below 

 being largest, those above smallest, the series graded from 

 one extreme to the other. The angular, lobed leaves 

 of the maple are seen to fit exquisitely into each other, 

 forming what has been well termed a " leaf mosaic." 

 The spirally arranged leaves of the beech and elm show 

 similar relations. The narrow leaves of many willows 

 cast little shadow and are borne more abundantly towards 

 the heart of the tree, than are those of the beech. Much- 

 divided leaves, like those of the hemlock or ragwort, allow 

 abundant light to penetrate to those placed below. 



One or two further examples may be given. The peri- 

 winkle (Plate III.) has both vertical and horizontal shoots. 

 The leaves are paired in the vertical shoot, the arrangement 

 is decussate, and the leaves stand out from the stem. In 

 the horizontal shoot the leaves lie in one plane and in two 

 rows, so that their origin in four rows is not obvious. If 

 we suppose an erect shoot laid on its side, then one pair 

 of leaves, which occupies the flanks, will reach its new 

 position by a twist of the short petiole through 90 degrees. 

 The leaves of the next pair, however, lie one above and one 

 below. The stalk of the former bends down and is also 

 bent horizontally through 90 degrees, that of the latter 

 bends up and also sideways. In these a complex movement 

 is required to bring the blades to the side of the stem. The 

 same result is achieved by Buddleia (Plate III.) and Phila- 

 delphus in a totally different fashion. In these plants the 

 leaves are again decussate, and in the horizontal branches 



