71 THE STRUCTURE OF ECONOMIC PLANTS 



The Foliage Leaf. — The foliage leaf may be regarded mor- 

 phologically as a lateral divergence from the axis, which, in the 

 majority of cases, is expanded and dorsi-ventrally flattened. Its 

 point of divergence is designated as a node, and usually one or 

 several buds develop in the axil of the leaf, which may be vegetative 

 or flower buds. In the former, the further development of the bud 

 results in a branch bearing foliage leaves in the same manner as the 

 primary axis from which it arises; while, in the latter, the bud 

 develops as an inflorescence. (See Chapter IV.) In other in- 

 stances, the bud may be dormant, developing later in the life of 

 the plant ; or, in the normal cycle of the plant, it may never develop 

 further. Less frequently, there is apparently no bud in the axil; 

 but it seems likely that the potentialities for bud development 

 reside in the tissue at this point, and under some circumstances 

 adventitious buds may be produced. 



Phyllotaxy. — The leaf arrangement or phyllotaxy of the foliage 

 leaves is often remarkably constant for a given species; and, con- 

 sequently, constitutes a helpful diagnostic character. There are 

 three principal phyllotaxies : alternate, in which a single leaf is 

 diverged from each node (Fig. xy); opposite, in which two leaves 

 are so diverged; and whorled, or verticillate, where there are three or 

 more leaves at a node. In the alternate phyllotaxy, the leaves are 

 arranged on the axis in a spiral; and because of the regularity of 

 the angular divergence (the angle formed between two successive 

 leaves) they are arranged in a definite number of ranks or rows. 

 The relationship is commonly expressed fractionally. Thus, in the 

 alternate arrangement, the Yi divergence is the characteristic one 

 for the Gramineae; the 3^ phyllotaxy is found commonly in the 

 Cyperaceae, sedges; and the 2^ arrangement is perhaps the most 

 common among dicotyledons. In this fractional expression of 

 phyllotaxy, the denominator indicates the number of leaves or 

 ranks in a complete cycle; and the numerator expresses the number 

 of times the stem is encircled to complete a cycle. In a % diverg- 

 ence, the cycle consists of five leaves and passes twice around the 

 stem, so that there are five ranks or rows of leaves; and, starting 

 at any given leaf, the sixth leaf lies in the same vertical plane as 

 the first leaf. In the opposite arrangement, the successive pairs of 

 leaves may lie in the same plane or at right angles to each other, 

 and the latter type of phyllotaxy is often referred to as decussate. 



Although the phyllotaxy of most plants is sufficiently constant 



