NATURE OF PLANTS 27 



large extent of the leaf surface and of the arrangement, structure 

 and modification of the leaves. The broad blades are devices for 

 gathering or absorbing as much of the sunlight as possible and 

 they increase the surface of the plant many hundreds of times. 

 Contrast the extent of surface of a tree in full foliage with the 

 bare branches of the winter. A mature maple develops annually 

 from one to two thousand square yards of leaf surface. 



One of the most noteworthy features about the leaves is their 

 arrangement or "hang" on the branches. In some plants they 

 are arranged in two^yertical.rows, in other instances three, four, 

 five, eight or more rows. By fastening a thread to a leaf and 

 winding it about the stem so as to touch the petiole of each suc- 

 ceeding leaf the arrangement of the leaves becomes more obvious. 

 Some plants have their leaves opposite in two rows or ranks, 

 others opposite in four ranks, each succeeding set of leaves being 

 at right angles to the lower set. This latter arrangement is called 

 decussate (Fig. 13). More commonly the leaves are spirally 

 arranged and the thread passed once, twice, thrice, five, eight, 

 etc., times around the stem before a leaf is reached that is exactly 

 over the one from which we started (Fig. 14). The number of 

 leaves passcLbefore .reaching one that stands over the first leaf 

 indicates the number of rows or ranks of leaves on the stem. This 

 variation in the arrangement of the leaves is simply a device to 

 bring the leaves into the light and prevent the shading of one leaf 

 by another. In opposite two ranked leaves each succeeding leaf 

 is over the one below it but the cutting off of the light is pre- 

 vented by lengthening the internodes and thus separating the 

 leaves. The decussate arrangement of the leaves is a more eco- 

 nomical arrangement because more leaves can be developed on 

 a given length of stem without danger of shading. The spiral 

 distribution of leaves is still a better device since each succeeding 

 leaf is placed a little above and to one side of the next lower leaf. 

 By this means the maximum of leaf surface can "be exposed to 

 the light in a given length of stem. No one can look at the leaves 

 of the maples, dogwoods, ailanthus, creeping and climbing vines, 

 etc., and not be impressed with the fact that the leaves are 

 arranged so as to catch the most favorable light without shading 



