82 MANUAL OF BOTANY 



round the stem, being at equal distances laterally from each 

 other. "When there are only two leaves in each whorl, they are 

 said to be qp^josiie. When leaves are opposite, the pairs as they 

 succeed each other usually cross at right angles, in which case 

 they are said to decussate {fig. 162), and the arrangement is 

 called dectissation. When different whorls succeed each other, 

 it also frequently happens that a somewhat similar arrangement 

 occurs ; thus the leaves of one whorl stand above the intervals 

 of the whorl below it. There are, however, commonly great 

 irregularities in this respect, and in some cases the number of 

 leaves in the successive whorls varies, by which their arrange- 

 ment becomes stiU more complicated. This is th« case, for 

 instance, in Lysimachia vulgnris. 



It sometimes happens that, by the non-development of the 

 internodes of an axillary branch, all the leaves of that branch 

 are brought close together, in which case they form a tuft or 

 fascicle {fig. 163), and the leaves are said to be tufted or 

 fascicled. Such an arrangement is well seen in the Barberry 

 and Larch. That the fascicle is thus produced is rendered 

 evident by the fact that in the. young branches of the Larch the 

 internodes afterwards become elongated and the leaves are then 

 separated from each other. 



1. Alternate Leaves. — Though this arrangement is often 

 termed scattered, it does not follow that there is necessarily any 

 irregularity in the position of the leaves. Indeed, it would be 

 better to term the arrangement spiral, as each leaf can be ascer- 

 tained to be placed upon an imaginary spiral line winding sym- 

 metrically round the stem, and to be at a distance from the next 

 leaf upon the spiral which may be expressed by a fraction of the 

 circumference of the stem. This distance, which is usually 

 constant, is known as the divergence of the leaves. The spiral 

 can be traced and the divergence ascertained by counting from 

 any particular leaf to the first one vertically above it. Thus if 

 we take a branch of the Apple or Cherry tree {fig. 164), and, 

 commencing witli any particular leaf which we will mark 1, 

 proceed upwards, taking the successive leaves in the order 

 of their ages and joining their bases by a line or piece of string, 

 we shall find that after passing the leaves marked 2, 3, 4, and 5, 

 we shall reach the one marked 6, which will be found to be 

 situated vertically over the 1st ; and then proceeding further, thaD 

 the 7th is directly over the 2nd, the 8th over the 3rd, the 9th over 

 the 4th, the 10th over the 5th, and the 11th over the 6th and 1st ; 

 «. vertical line drawn down the stem through the base of anj' 



