from above it will be seen that the leaves are arranged in a series of 

 fairly distinct ranks. The number of these ranks is important, since 

 it has a direct relation to leaf form. The greater the number of ranks 

 the narrower the leaves. The smaller the number of ranks the broader 

 the leaves. Facts evidently explained by our conception of the leaf as 

 the organ of light relation. 



Thus far it has been assumed that the leaf-bearing stem has been 

 erect. If by any chance, or by the necessities of growth it should 

 change from the erect to the horizontal position, it is evident that to 

 secure proper light relations the leaf position must also change. Com- 

 parisons of leaf positions upon erect and horizontal stems taken from 

 the same plant will prove of great value in emphasizing the fact that 

 above all other things the leaf must have light exposure. 



Material foe Illustration : 



Erect and horizontal stems of elm, maple, linn, oak, apple, peach, 

 cherry, catnip, wild pinks, honeysuckle or of any plant that may be 

 growing near at hand. 



I have considered as yet only cases in which the leaves were entire, 

 or with unbroken margins, since these furnished the simplest illustra- 

 tions. In the case of lobed or dissected leaves, the conditions are 

 somewhat different. In the simpler forms of lobed leaves, the lobing 

 is evidently a device to prevent the shading of underlying leaves. If 

 you recall the ordinary ivy with its sharply angled leaves, almost geo- 

 metrical in their regularity, this fact will be evident. If a growing tip 

 of this plant, as it clings to the wall, be carefully flattened down it will 

 be seen that the leaves iit into each other so accurately by means of 

 these angles, that on the one hand there is scarcely any perceptible 

 shading, and on the other there is scarcely any space unoccupied by 

 the leaf. Such accurate fitting of leaves when brought to a common 

 plane, produce what is known as leaf "mosaics," which simply serve to 

 again prove that the leaf is the organ of light relation. Where the 

 leaves are much dissected, as in the case of the common rag-weed,there 

 is the same arrangement in ranks, the same arrangement of leaves in 

 different planes as in the case of the entire leaf, but as a rule no 

 marked diminution in the size of the leaves as we pass from the base 

 to the top of the plant, the constant shifting of the parts of the dis- 

 sected leaf, and the possible play of light through the openings be- 

 tween the leaf parts, being sufficient to prevent any portion of the 

 underlying leaf from being continuously shaded. 



