512 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



horizontally growing boughs of the Elm, Ivy, or Laurel will be seen to 

 have their leaves in two ranks, so as to expose their surfaces to light from 

 above, i.e. they are represented by h But if shoots of the Laurel, for 

 example, grow vertically upwards out of the summit of the bush, the 

 leaves will be found arranged all round the shoot according to the § plan. 



Sometimes this \ arrangement is only imitated, as by the Yew. 

 Young shoots — or all of them in the Irish Yew — stand erect, the leaves 

 being scattered all round them according to the § plan ; on the hori- 

 zontal shoots the leaves stand in two ranks, but only in consequence of a 

 twist at the base of the little footstalk. 



The next question is, what is the origin of these two systems, " opposite " 

 and " alternate " ? As the two cotyledons of all dicotyledons are opposite 

 in position, they suggest that "opposition" in leaves was the primitive 

 condition, and that alternate arrangements are later and derived. 



Numerous changes can be well studied among the stems of the 

 Jerusalem Artichoke. It often happens that opposite pairs of leaves 

 below, and alternate ones above, can be found on one and the same stem. 

 The change can be there readily observed. The two leaves of a pair will 

 be slightly separated in height. Then, those of the next pair a little more 

 so. At the same time a slight tendency to shift the position of the leaves 

 round the stem will be noticed. The third pair will thus have not 

 only a large internode between their two leaves, but be much less in an 

 "opposite " position. 



After a time, enough spaces between the leaves of each pair will be 

 present to establish them as " alternate," but after some irregularities the 

 sixth leaf finds itself over the leaf which may now be selected as No. 1. 



An important fact must now be noticed. Suppose, on holding the 

 stem erect, the pair of lower leaves mil be standing right and left of the 

 observer ; then, if it be the right-hand leaf of a pair which is a little raised 

 above the common level of the two, it will always be the right-hand 

 one which rises above its companion, i.e. on rotating the stem. Hence a 

 continual spiral line can be drawn through the now separated leaves. 



The following diagram will explain this, where 1,2; 3, 4 ; 5, 6 ; &G., 

 stand for the original pairs of leaves, each pair standing at right angles 

 to the pairs above and below. The numbers show the order of " up- 

 lifting." 



The horizontal shifting is suppressed to make matters clearer. 



<-2 

 5 

 10 

 13 

 18 

 21 



3 8 11 1G 19 20 15 12 7 4 

 22 

 17 

 14 



9 



6 



1 



