CHLOROPHYLL AND LIGHT INTENSITY. 381 



stripe is to be seen. This turning movement of the chlorophyll body is very 

 quickly performed, and can be repeatedly effected by darkening and illuminating 

 the cells of the Mesocarpus filament. 



In cells, too, which are joined together to form tissues, this displacement and 

 movement of the chlorophyll-granules often appears. It has been noticed for a 

 long time that in the prothallium of ferns, in the leaf-like liverworts, in the 

 leaflets of many mosses, and even in the large, delicate foliage-leaves of flowering 

 plants, the green tissue appears to be coloured a lighter or darker green according 

 to the intensity of the incident light; that under the influence of intense sunlight 

 they become blanched and yellowish-green, but in weak light assume a darker tint. 

 If a strip of black paper be placed on a foliage-leaf, illuminated by the sun, so that 

 only a portion of the leaf -surface is covered by it, and if the paper be removed after 

 a short time, the portion left uncovered and illuminated by the uninterrupted rays 

 of the sun appears light green, while that part on which lay the strip of paper, and 

 from which the sun's rays were withheld, is dark green. Careful investigations 

 have shown that this change of colour is due to displacement of the chlorophyll- 

 granules. In diffuse light the chlorophyll-granules group themselves on those cell- 

 walls on whose surface the light falls perpendicularly, and consequently in the 

 cylindrical palisade cells of the foliage-leaf on the small walls parallel to its upper 

 surface, and it is clear that such cells (and therefore the tissue formed by them) 

 have a dark-green appearance when looked at in the direction of the incident light. 

 As soon as they are illuminated by direct sunlight, the chlorophyll-granules retire 

 from these walls and take up their position on the cell-walls which are parallel to 

 the direction of the incident light. In palisade cells the chlorophyll-granules group 

 themselves by the side of the long lateral walls, while the short walls, which are at 

 right angles to the rays, are colourless and free from chlorophyll. In the branched 

 cells of the spongy parenchyma the chlorophyll-granules, which in diffuse light 

 were equally distributed in the cell, heap themselves together in groups in the 

 branches, while the central portions of the cells become clear and free from 

 chlorophyll. The whole tissue, however, in which this displacement has been 

 completed appears much paler than before, and displays usually a decidedly 

 yellowish-green tint. This change of position of the granules, according to the 

 intensity of illumination, may be particularly well seen in the very simply 

 constructed leaf-like duckweed, especially in Lemna trisulca. Three sections of 

 the green tissue of this plant, vertical to the surface, are shown in fig. 97. 



With these phenomena is indeed also connected the alteration of shape which 

 is observed under varied illumination in chlorophyll-granules. In the leaflets of 

 Funaria hygrometrica, a moss very common on piles of charcoal, damp walls, and 

 rocks, the chlorophyll-granules, which are close to the outer walls of the cells, are 

 flattened out, angular, and comparable to small polygonal tablets, in diffuse light. 

 They are also so arranged that the entire wall covered by them appears an uniform 

 green, and only narrow, colour-less lines remain between them. As soon as direct 

 sunlight falls on them they quickly alter their shape, the tablets becoming hemi- 



