THE AXGIOSPERMAE : LEAVES 9S5 



only the cotyledons but the first one or two pairs of epicotyledonous leaves 

 are frequently opposite, while the later leaves are formed singlv at the nodes. 

 Such conclusions are, however, of strictly limited application. 



One of the most general rules of construction in radial axes is that of 

 the equidistance of parts, which holds good not only for outer members but 

 also for internal structures such as vascular bundles, 'i'his rule is followed 

 both in the opposite and in the whorled phyllotaxis. Opposite pairs of leaves, 

 moreover, usually alternate at right angles with those above and below them, 

 thus forming four ranks, the decussate arrangement. Paired leaves in two 

 ranks only, the distichous arrangement, are relatively rare, though single 

 leaves alternating in two ranks are common in Monocotyledons and are 

 almost universal in the great families of the Iridaceae and Orchidaceae. 



The opposite and the whorled phyllotaxis are closely similar in that they 

 imply svmmetrical, radial growth at the apex with equal spacing of the 

 primordia, and the paired condition may be looked upon as simply a whorl of 

 two members. As both, except for the bilateral distichous cases, imply radial 

 symmetrv of growth at the apex, they are commoner in orthotropic than in 

 plagiotropic shoots. A tendency towards the development of bilateral or 

 even dorsiventral symmetry may, however, often be observed in the growth 

 of the axillary buds of such leaves, when the leaves themselves do not show 

 it. For example, all the axillary shoots on one side may be stronger than 

 those on the other side of the axis, or may become flowers while the buds 

 standing opposite develop vegetatively. This produces a secondary dorsi- 

 ventrality even in an orthotropic shoot and is usually associated with the 

 presence of truly dorsiventral shoots in members of the same family. In other 

 cases, strong and weak shoots may alternate on each side of the main axis, thus 

 showing a tendencv towards spiral organization. The close relationships of 

 opposite and whorled arrangements is shown by their tendency to pass one 

 into the other. Individuals can often be found, of species which normally 

 bear opposite paired leaves, with three or even four leaves at each node, or 

 sometimes only at certain nodes. The essential similarity of the two conditions 

 lies in the simultaneous formation of several primordia at the apex, but the 

 number actuallv formed seems to bear some relation to the vigour, if not 

 the size, of the apex, since it tends to increase as the plant matures. It should 

 be noted that in whorls of several leaves, as in those with only two, the members 

 of each whorl alternate in position with those of the whorls above and 



below. 



The rule of equal spacing applies also in the cases of leaves inserted 

 singly, which are by far the more numerous. If we examine a shoot of this 

 kind we shall see, that if we start from any leaf and trace a line from this to 

 the next leaf above it and so on until we come to a leaf which is exactly above 

 the first, in a vertical line, we shall have traced a spiral round the stem. We 

 shall find, further, that the number of leaves intervening between any two 

 in the same vertical line is constant, and so is also the number of turns of 

 the spiral round the stem in passing from one to the other. This means that 

 successive leaves must be equally spaced around the stem, or in other words, 

 32 A 



