NATURE OF PLANTS 25 



been shown that a membrane pierced by sufficiently small open- 

 ings which are separated from one another by a distance of at 

 least ten times their diameter, permits dififusion of gases as readily 

 as though there were no membrane at all. The structure of the 

 epidermis is an admirable example of such a perforated mem- 

 brane and it is so perfectly adjusted to the work in hand that it 

 could accomplish much more than the necessities of the plant 

 demand. 



14. Noteworthy Features of Leaves. — Now that some idea has 

 been gained of the nature and extent of the work performed by 

 the leaves we are prepared to comprehend the meaning of the 

 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 su'-face 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 rows, in other instances three, four, five, 

 eight or more rows. By fastening a thread to a leaf and wind- 

 ing it about the stem so as to touch the petiole of each succeeding 

 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 passed before 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- 



