Page:The New International Encyclopædia 1st ed. v. 12.djvu/58

* LEAF. 46 LEAF. leaves, therefore, proinincnt among other foliar organs are the following: Pitchers, as in the various 'pitcher plants' for entrapping various insects; sensitive fly-traps, as in sundews and Diona;a (q.v. ) (see Cakkivorous Plants) ; storage organs, as in hulh-scales. and many seed leaves (cotyledons) ; bud-scales, used for pro- tecting young parts; spines, as in the barberry and liolly. where every gradation between spiny- toothed k'aves and true spines is found ; ten- drils, which are often leaves or leaf-parts adapted for climbing: braets, which are leaves modified in size and color and associated with flowers; the /loral organs, as sepals, petals, stamens, car- pels, all of which may be regarded as foliar struc- tures, hut ])roI)ably not modified foliage leaves as is commonly stated. Arraxoement of Leaves. The distribution of leaves on the stem has given rise to a subject called ])hyllotaxy, which undertakes to study the laws which govern the distribution. Thfc general conclusion reached is that leaves are distrilmted so as to economize space and to ob- tain a liglit exposure. l)ut this is to be regarded as the result of the arrangement rather than its cause. The most fundamental classification of leaves on the basis of arrangement is into the cyclic and spiral arrangements. In the former, two or more leaves stand together at the same joint (ncKle) of the slem. dividing the circum- ference between them. If the cycle consists of two leaves they are called opposite, while if it consists of three or more they are called vvhorled or verticillate. In the spiral arrange- ments the leaves stand singly one after an- other — that is, each point of the stem bears but a single leaf, and they are commonly spoken of as alternate. It is the spiral arrangement ■which has developed the largest discussion in reference to the laws of pliyllotaxy, for the cyclic arrangement rc])rcsents merely two or more spirals ascending the stem. In the simplest alter- nate arrangement the second leaf stands upon the opposite side of the stem from the first, and the third leaf stands directly over the first. Tliis results in two vertical rows of leaves, one on each side of the stem, an arrangement indicated by the fracticm one-half. The whole fraction signi- fies the angular divergence between two successive leaves, the denominator the number of vertical rows. The next higlier ari'angement is one in which the angular divergence between two suc- cessive leaves is one-third of the circumference, and as a consequence the leaves occur in three vertical rows, and the fractional expression is one-third. The next higlier arrangement is indi- cated by the fraction two-fifths, which means that the angular divergence is two-fifths of the circum- ference of llic stem, that there are five vertical rows, and that the spiral line makes two turns around the stem before it reaches the same vertical row with which it started. The curious feature of the sy.steni appears at this point. Succeeding fractions may be obtained by adding the numerators and denominators of the two pre- ceding fractions. For example, the fraction which follows the one-half and one-third arrange- ments is two-fifths, and the next would be three- eightlis, and so on. The higher numbers, such as five-thirteenths, eight twenty-firsts, etc., oc- cur in certain pine-cones, but in ordinary foliage leaves the lower numbers of the series are the common ones. It is often diflfieult to determine the normal arrangement, since the stem axis is not always perfectlj' straight in its growth. Ecology of the Leaf. Ecologically, the leaves of plants may be considered from the standpoint of the various leaf-forms found in nature, and (2) from that of the evidence nlj tained by exiieriments to determine the cau^i of the various forms. In general, leaves ar.- expanded organs, and they also liave a ])o^i lion which is in most cases perpendicular to the majority of incident rays of light. Tliis It can readily be seen that these conditions favor the absorption of the largest quantities of radiant energy; and, inasmuch as radiant energy is esscNtial to the growth and life of plants, it is clear that the expanded form and the diaheliotropic position are distinct- FlQ. 4. BRAKCH OF HABEBELL {Campanula rotunrlifoli.i) which has suffered injury, and has in consequence developed basai (juvenile) leaves upon the stem. This result has also been ascribed to the illumi- nation. ly advantageous, though it by no means follows that the need for light has caused either the form or the position. Large numbers of leaves are finely divided; this is conspicuously true in the hydrophytes (q.v.), but it is also true of a vast number of plants with aerial leaves. While we can hardly believe that compound leaves have • been caused in any such way. it is nevertheless true that ,a larger amount of leaf surface can be presented to the sunlight than in the case of plants with entire leaves. Perhaps the most I
 * )osition is technically termed diahcliotro|)ic.