i] OPPOSITE LEAVES 7 



In marked contrast to the above are the cases of 

 opposite leaves, where we find two opposed leaf-insertions 

 at each node of the axis. Common examples are met 

 with in the Lilac, Elder, Honeysuckles and other members 

 of the natural order (Caprifoliacese) to which these latter 

 plants belong ; also in the Maples, Horse-chestnut, Ash, 

 Cupressus and Thuja, and numerous others, in some cases 

 including whole natural orders. 



In the vast majority of such cases, including all those 

 quoted, the pairs of opposite leaves are so arranged that 

 the longitudinal axis of each opposite pair is at right angles 

 to that of each pair above and below, and this kind of 

 phyllotaxy is universally known as decussate. We may 

 here again also speak of the vertical rows of insertions as 

 orthostichies, and then we have four such orthostichies in 

 decussate phyllotaxy. There are very few cases among 

 foliage-leaves where the opposite insertions are superposed 

 on erect stems, e.g. some Euphorbias. Loranthus europceus 

 and Potamogeton densus may possibly furnish exceptional 

 cases, but in the latter the branches are floating and not 

 erect, a fact in obvious relation to the need for the plant 

 to expose its leaves to the light. 



It is also clearly possible to regard opposite leaves 

 as only a particular case of whorled leaves, that is where 

 several leaf-insertions are situated at the same node 

 or level of the axis ; in practice, however, it is usual 

 to limit the term whorled to cases where at least three 

 insertions (e.g. Juniper) are at the same level on the 

 axis. 



Examples of whorled (or verticillate) phyllotaxy are 

 the following. The Oleander, Elodea canadensis, and 

 Junipers, have each usually three leaves in the whorl ; 

 Lysiniachia quadrifolia, Paris quadrifolia, Myriophyllum 

 sjiicatum, Heather, &c, have usually four. In the various 



