Fig. 110.—Phyllotaxy of Box Elder, Elm, Apple.
There are several kinds of alternate arrangement. In the elm shoot, in Fig.[ 110], the third bud is vertically above the first. This is true no matter which bud is taken as the starting point. Draw a thread around the stem until the two buds are joined. Set a pin at each bud. Observe that two buds are passed (not counting the last) and that the thread makes one circuit of the stem. Representing the number of buds by a denominator, and the number of circuits by a numerator, we have the fraction ½, which expresses the part of the circle that lies between any two buds. That is, the buds are one half of 360 degrees apart, or 180 degrees. Looking endwise at the stem, the leaves are seen to be 2-ranked. Note that in the apple shoot (Fig. [110], right) the thread makes two circuits and five buds are passed: two fifths represents the divergence between the buds. The leaves are 5-ranked.
Fig. 111.—Phyllotaxy of the Potato Tuber. Work it out on a fresh long tuber.
Every plant has its own arrangement of leaves. For opposite leaves, see maple, box elder, ash, lilac, honeysuckle, mint, fuchsia. For 2-ranked arrangement, see all grasses, Indian corn, basswood, elm. For 3-ranked arrangement, see all sedges. For 5-ranked (which is one of the commonest), see apple, cherry, pear, peach, plum, poplar, willow. For 8-ranked, see holly, osage orange, some willows. More complicated arrangements occur in bulbs, house leeks, and other condensed plants. The buds or “eyes” on a potato tuber, which is an underground stem (why?), show a spiral arrangement (Fig. [111]). The arrangement of leaves on the stem is known as phyllotaxy (literally, “leaf arrangement”). Make out the phyllotaxy on six different plants nearest the schoolhouse door.
In some plants, several leaves occur at one level, being arranged in a circle around the stem. Such leaves are said to be verticillate, or whorled. Leaves arranged in this way are usually narrow: why?
Although a definite arrangement of leaves is the rule in most plants, it is subject to modification. On shoots that receive the light only from one side or that grow in difficult positions, the arrangement may not be definite. Examine shoots that grow on the under side of dense tree tops or in other partially lighted positions.
Suggestions.—55. The pupil should match leaves to determine whether any two are alike. Why? Compare leaves from the same plant in size, shape, colour, form of margin, length of petiole, venation, texture (as to thickness or thinness), stage of maturity, smoothness or hairiness. 56. Let the pupil take an average leaf from each of the first ten different kinds of plants that he meets and compare them as to the above points (in Exercise 55), and also name the shapes. Determine how the various leaves resemble and differ. 57. Describe the stipules of rose, apple, fig, willow, violet, pea, or others. 58. In what part of the world are parallel-veined leaves the more common? 59. Do you know of parallel-veined leaves that have lobed or dentate margins? 60. What becomes of dead leaves? 61. Why is there no grass or other undergrowth under pine and spruce trees? 62. Name several leaves that are useful for decorations. Why are they useful? 63. What trees in your vicinity are most esteemed as shade trees? What is the character of their foliage? 64. Why are the internodes so long in water-sprouts and suckers? 65. How do foliage characters in corn or sorghum differ when the plants are grown in rows or broadcast? Why? 66. Why may removal of half the plants increase the yield of cotton or sugar-beets or lettuce? 67. How do leaves curl when they wither? Do different leaves behave differently in this respect? 68. What kinds of leaves do you know to be eaten by insects? By cattle? By horses? What kinds are used for human food? 69. How would you describe the shape of leaf of peach? apple? elm? hackberry? maple? sweet-gum? corn? wheat? cotton? hickory? cowpea? strawberry? chrysanthemum? rose? carnation? 70. Are any of the fore-going leaves compound? How do you describe the shape of a compound leaf? 71. How many sizes of leaves do you find on the bush or tree nearest the schoolroom door? 72. How many colours or shades? 73. How many lengths of petioles? 74. Bring in all the shapes of leaves that you can find.
