128 THE BIOLOGY OF FLOWERING PLANTS 



epiderm, despite the fact that it is directly exposed to the 

 atmosphere, is very poorly supplied with carbon dioxide. 



The chloroplasts lying in the peripheral plasma are not 

 always fixed in position. Especially in the lower plants 

 movements under the influence of different degrees of 

 illumination are common. Among flowering plants move- 

 ments of the chloroplasts in the fronds of Lemna, the duck- 

 weed, are best known. In light of moderate intensities the 

 chloroplasts lie on the upper and lower walls of the cells ; 

 mutual shading is here not serious, as the frond has only 

 two layers of chlorenchyma. In direct sunlight the chloro- 

 plasts move to the side walls, and so take up a profile position 

 to the sun's rays. At night the chloroplasts are more 

 scattered. In the palisade of ordinary leaves the chloro- 

 plasts do not move, but movements occur in the spongy 

 parenchyma. In direct sunHght there is a tendency for the 

 chloroplasts to gather in masses in the shade of the overlying 

 palisade, and this may lead to an alteration in the colour of a 

 leaf. Thus an elder leaf growing in the shade has a deep 

 green colour ; if it is exposed to direct sunlight it becomes 

 lighter green. 



§ 14. Chlorophyll and the Absorption of Light 



The green colour of the chloroplast is due to the presence 

 of a group of four pigments, the composition of which has 

 been the subject of a great deal of research. Recently, the 

 work of Willstatter (19 13) and his collaborators has greatly 

 advanced our knowledge and put it on a sound chemical 

 basis. There are present two bright green pigments, named 

 chlorophyll a and chlorophyll b, and two yellow pigments, 

 named carotin and xanthophyll. The two latter occur 

 widely, apart from chlorophyll, in fruits and flowers of 

 yellow and orange colour. Alone they are incapable of 

 carrying on assimilation ; it is not known whether they play 

 a definite part in the process in conjunction with the chloro- 

 phyll proper, though many attempts have been made to 

 show that they do. The four pigments are present in the 



