120 THE BIOLOGY OF FLOWERING PLANTS 



which causes the leaf to move, and determines the direction 

 of the movement. This response is termed phototropism. 

 In leaves, and in leaflets, which possess joints or pulvini, 

 the movement is carried out by the joint, and is due to 

 changes in turgor pressure. The leaves of a scarlet runner, 

 for example, have a pulvinus at the base of each leaflet, 

 and another at the base of the stalk. Each leaflet of a 

 clover or of a wood-sorrel leaf possesses a pulvinus. The 

 pulvini remain capable of movement long after the leaf is 

 mature, and the leaf may at any time readjust itself to new 

 conditions of illumination. A potted scarlet runner placed 

 near a window brings all the leaf blades, by bendings and 

 twistings in the joints, to face the light ; if it is reversed, 

 then in the course of a few hours it readjusts the whole 

 light-absorbing surface. The turgor movements in the 

 pulvini are completely reversible, and may be repeated 

 almost indefinitely. 



Most leaves, however, possess no pulvini, and then the 

 movement is carried out by differential grovvth on the 

 different sides of the petiole — particularly near its base and 

 apex. Such growth movements can only take place in the 

 growing organ, and they commonly cease or become very 

 sHght as the leaf comes to maturity. In some cases, e.g. the 

 garden nasturtium or the house geranium, the petiole 

 may remain capable of growth for a long time. As a rule, 

 however, such leaves assume a fixed light position, though 

 this phrase must not be taken in too rigid a sense. Now the 

 direction from which maximum light strikes the plant varies 

 throughout the day, most markedly so in sunshine, and a 

 fixed position cannot, therefore, be assumed in relation 

 to sunlight. It has been found that it is the direction from 

 which comes the maximum diffused light that determines, 

 in the main, the leaf position of ordinary plants. It may, of 

 course, frequently occur that this is also the position in 

 which the maximum of direct sunlight falls on the leaf. 

 We have said that the plants of temperate climates have to 

 work for the most part in diffused light, and this conclusion 

 may be extended to cover all plants which grow in 



