306 ECOLOG V. 



wood which has a large percentage of fibrous elements, and in which the 

 ducts are comparatively small. Wood with numerous large vessels is also 

 more spongy, and therefore lighter than woods with a close fibrous struc- 

 ture. We should find it an exceedingly interesting study if we made a 

 comparative examination of the growth and strength of the different woods. 



579. Phyllotaxy, or arrangement of leaves. — In our study of the organs 

 which utilize carbon for food, and in examining buds on the winter shoots of 

 woody plants, we could not fail to be impressed with some peculiarities in the 

 arrangement of these members on the stem of the plant. Even in the liver- 

 worts and mosses we note that where there is any indication of leaf-like 

 expansions on a central axis there is a general plan of arrangement of these 

 leaf-like structures over successive zones of the axis. 



In the horse-chestnut, as we have already observed, the leaves are in pairs, 

 each one of the pair standing opposite its partner, while the pair just below or 

 above stand across the stem at right angles to the position of the former pair. 

 In other cases (the common bed straw) the leaves are in whorls, that is several 

 stand at the same level on the axis, distributed around the stem. By far the 

 larger number of plants have their leaves arranged alcernately. A simple ex- 

 ample of alternate leaves is presented by the elm (fig. 347), where the leaves 

 stand successively on alternate sides of the stem, so that the distance from one 

 leaf to the next, as one would measure around the stem, is exactly one half 

 the distance around the stem. This arrangement is 1/2, or the angle of diver- 

 gence of one leaf from the next is 1/2. In the case of the sedges the angle of 

 divergence is less, that is 1/3. 



By far the larger number of those plants which have the alternate arrangement 

 have the leaves set at an angle of divergence represented by the fraction 2/5. 



580. Other angles of divergence have been discovered, and much stress has 

 been laid on what is termed a law in the growth of the stem with reference to 

 the position which the leaves occupy. There are, however, numerous excep- 

 tions to this regular arrangement, which have caused some to question the 

 importance of any theory like that of the ' ' spiral theory ' ' of growth propounded 

 by Goethe and others of his time. 



581. As a result, however, of one arrangement or another we see a beauti- 

 ful adaptation of the plant parts to environment, or the influence which envi- 

 ronment, especially light, has had on the arrangement of the leaves and 

 branches of the plant. Access to light and air are of the greatest importance 

 to green plants, and one cannot fail to be profoundly impressed with the work- 

 ings of the natural laws in obedience to which the great variety of plants have 

 worked out this adaptation in manifold ways. 



