124 THE BIOLOGY OF FLOWERING PLANTS 



is exposed to the sun's rays. This may be seen in our native 

 wood-sorrel, and in other species of Oxalis. Through 

 the day, in its natural shaded habitat, the three leaflets of 

 Oxalis are spread out at right angles to the petiole, but 

 if the plant is exposed to direct sunlight they droop and fold 

 together, assuming the position that is taken up normally 

 in the dark every night. These leaflets move by pulvini, 

 and the drooping in the sun is connected v^^ith loss of turgor. 

 Similar movements are carried out by the leaflets of Acacias 

 and of many other plants. Robinia, the false acacia, is a 

 good example of this behaviour ; in the morning the leaflets 

 are horizontal, but, as the sun rises about 30 degrees above 

 the horizon, they move up and assume a vertical position. 

 In many plants, particularly those of very hot, dry climates, 

 the leaves assume a fixed light position of this nature, 

 hanging vertically, and with the edges turned in the direction 

 of maximum insolation. This profile position is shown by 

 Eucalyptus globulus and many other members of this Aus- 

 traHan genus. The famous " compass plants " also show 

 this relation. Of these the European Lactuca Scariola is 

 the best known. Its leaves are borne in | spiral phyllotaxy, 

 and, according to Neger, in shady places, and in high 

 latitudes as in Norway, they maintain this arrange- 

 ment. Where they are exposed to intense sunlight, they 

 twist so that they come to stand in two rows, with their 

 edges pointing north and south. So constant is this arrange- 

 ment in some compass plants of the American prairies, 

 e.g. Silphimn laciniatum, that they are said to have been 

 utilised by the plainsmen in finding their way. The same 

 feature is shown by the leaves of many tropical forest trees, 

 e.g. Amherstia ?iobilis, which when young and tender hang 

 vertically, and later, as they harden, take up a horizontal 

 position. The flat segments of some cactuses, e.g. Opuntia 

 rnay stand north and south. We might also regard the 

 grass type of leaf, especially in extreme forms such as that 

 of the Iris, as escaping direct insolation. The spruce bears 

 its needles erect on exposed branches, and horizontal when 

 shaded. 



