114 Botany

right angles to that immediately above and below it. This arrangement is called decussate. Where leaves are placed in a ring round the stem, as in cleavers, the arrangement is said to be whorled, and where neither whorled nor opposite it is alternate. Compare the number of rows of leaves on a willow branch with that on the stem of a tobacco plant. These two plants will illustrate another principle concerned with shade prevention. Where the leaves are long, as in the tobacco, the internodes of the stem are also long so that there shall be no overlapping, wheras short leaves , as in the willow, are set at short intervals. By an adjustment in the length of the leaf petioles, shading may be avoided. The petioles of the lowest leaves are often long, carrying them outside the shade of the foliage on the upper part of the stem. A similar arrangement is seen in many rosette plants such as the dandelion and cat'sear (Fig. 78). In other cases the lowest leaves grow outwards horizontally from the stem while those above grow more vertically. This is the case with the buttercup. The leaves springing from the base (the radical leaves) have long petioles and spread horizontally. Shading of these is prevented in two ways: by the reduction in size of the upper leaves, and by the fact that these latter spring more ver- tically from the stem. The woolly mullein furnishes an even better example. In the case of compound leaves (i.e. leaves, in which, like the bean, the blade is divided into several parts) there is not generally reduction of the upper leaves or lengthening of the lower petioles, since, at some time or other during the day, the sunlight is able to make its way through the broken blades as they move to and fro in the breeze. In creeping stems, as a rule, every leaf assumes the horizontal position. In the periwinkle (Figs. 76- 77) for instance, the arrangement of the leaves is,