PHOTOTROPISM 655 



"stimuli," and the so-called "sleep" movements of the leaves and leaflets of 

 many other legumes. The opening and closing of stomates and the movements 

 of leaves caused by wilting and recovery illustrate turgor movements that are 

 not associated vt^ith pulvini. 



3. Hydration Movements. — Movements may occur in non-living plant 

 tissues or organs as a result of the hydration or dehydration of the cell walls. 

 These movements are illustrated by the splitting of pods, the opening of 

 capsules, and the rapid movements of mature fern sporangia. Movements of 

 these kinds are not produced by the physiological activities of living cells and 

 will not be considered further in this discussion. 



Phototropism. — Phototropic curvatures are familiar to all close observers 

 of plants. They are particularly conspicuous in plants growing in situations 

 in which they are exposed to unequal illumination on opposite sides. Under 

 such circumstances the growing stems usually bend toward the direction of 

 the more intense light and the leaves also become definitely oriented with rela- 

 tion to the light source, regardless of the position of their attachment to the 

 stem. When vines such as the English ivy {Hedera helix) are growing on a 

 wall so that light strikes the plants mainly from one direction the leaf blades 

 occupy practically the entire exposed surface with a minimum of overlapping. 

 The leaf blades appear to fit together so exactly that the resulting patterns 

 are known as "leaf mosaics." Similar, though less accurately formed "leaf 

 mosaics" are present in most plants bearing large numbers of leaves. Any- 

 one standing beneath a large maple or oak tree, for example, cannot fail to 

 be impressed by the completeness with which the sky is obscured by the leaf 

 pattern. The leaves of some herbaceous plants {Lactucaj Silphium, etc.) are 

 often so oriented that the blade surfaces face the east and west and only the 

 edge of the blade receives the full intensity of the mid-day sun. This orienta- 

 tion is so conspicious that these plants are commonly known as "compass 

 plants." The leaf blades of the turkey oak {Quercus catesbei) also have a 

 characteristic vertical orientation under conditions of intense sunlight. 



The movements which bring the leaves and stems into the oriented posi- 

 tions just described are caused by differences in the growth rates on the ex- 

 posed and shaded portions of the stems and petioles. In some species these 

 different rates of growth are sufficiently rapid to become conspicuous. The 

 flowering heads of young sunflowers, for example, face the east in the morn- 

 ing and follow the sun during the day. As soon as growth of the stem ceases 

 the movement likewise ends. 



While growing stems and leaves usually react positively to unilateral 

 illumination, roots commonly show no response. However, some roots such as 

 those of white mustard {Brassica alhn) , are negatively phototropic as are the 



