RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. 409 



All the rows of leaves, whether there be three, five, eight, or thirteen of them, 

 are equally illumined by the sunbeams which strike them from above in the 

 direction of the axis of the branch; no row throws another into the shade, and only 

 the upper individual members of a row standing above one another can deprive the 

 lower members of light. But even this shading is avoided, chiefly by the adapta- 

 tion of the length and direction of the foliage-leaves to the height of a story. 



If the stories are low, so that the consecutive leaves of a rectilineal row are 

 separated only by short distances, then the leaves are short; if the stories are high, 

 then the leaves are long; the length is always so arranged that the sunbeams can 

 penetrate into the space between every two leaves of a row, and can, so to speak, 

 illuminate the interior of the story. 



It should be remembered here that the sun does not shine down vertically upon 

 branches having an upward direction, that its rays, even at the equator, fall 

 obliquely in the morning and evening, and at these times illuminate the space, 

 bounded above and below by two consecutive leaves of a row, just like the rays of 

 the rising and setting sun which enter a room through the window. But this does 

 not say that no leaf is thrown into the shade throughout the entire day. This 

 would be impossible, from the fact that the sun's rays at each hour of the day fall 

 at a different angle on the plants which remain firmly fixed and immovable in the 

 soil. The leaves of one side are partially shaded in the morning, and those of the 

 other side, in the afternoon; or they are only illuminated by diffuse light; and the 

 upright stem, which is set round about with projecting leaves, must necessarily 

 shade a portion of them for a short time during the day. But these shadows, like 

 the dark lines thrown by the gnomon of a sun-dial, must continually move forward 

 with the sun, and only remain in one place for a little while. 



The entrance of the sun's rays between the leaves situated above one another is, 

 moreover, materially influenced by the direction of the leaf -blades. A leaf pro- 

 jecting obliquely upwards from the stem, with its midrib in the plane of the 

 incident rays, will not at any hour of the day deprive its lower-placed neighbour of 

 too much light, or at any rate to a much less extent than will a leaf whose blade is 

 extended horizontally or sloped a little in an outward direction, and which presents 

 its broad side to the incident sunbeams. This explains a phenomenon which is 

 .seen very often in annual and biennial composites and crucif ers with straight, erect 

 rstems. The lowest leaves of these plants form a right angle with the axis of the 

 stem, and lie with their broad surfaces on the soil, completely covering a larger or 

 smaller area. These can obviously not take away the light from any other leaves 

 of the same plant. The leaves inserted higher up the stem are, on the other hand, 

 no longer extended horizontally, but rather in an upward direction, and form an 

 single with the stem which is less than a right angle; and the highest leaves even 

 approach the upright, their midribs lying in the plane of the incident noonday 

 rays. 



In accordance with this adjustment, an alteration of the dimensions, particularly 

 of the length of its leaves, may be observed at different heights of an erect thickly- 



