ORGANS OF KUTRITION. 141 



variety of others also occur, which become more complicated as 

 the mimber of leaves &c., in the spire is increased; but in 

 those cases, where the leaves, &c., are so j,. ^ 



numerous as to be close to each other, '^ " ' 



as in the Screw-pine, in the Pine-apple 

 {fig. 706, 2 ), and in the fi-uit of Coniferous 

 Plants (Jig. 269), the spiral arrangement 

 is at once evident. The table in the follow- 

 ing page, slightly altered from M. Braun, 

 and taken from Balfour's Class-Book of 

 Botany, will exhibit the more common 

 modes of angular divergence in leaves, 

 and their modifications. 



"We have thus endeavoured to show 

 that when leaves are alternate, these 

 taken as a whole form a spiral round 

 the axis. The spire mav either turn ^io- 269. Cone or fruit 

 from right to left, or from 'left to right. (LS^vSS ^"^ 

 In the majority of cases, the direction 



in both the stem and branches is the same, and it is then 

 said to be homodromous ; but instances also occasionally occur 

 in which the direction is ditferent, when it is called heterodro- 

 mous. 



Besides the series of spirals which have been alluded to, 

 others also occur, as \, \, §, f:,, Z,, &c. ; also l, j, 3, |, &c.; and 

 others are also met with; these, however, are all of more or less 

 rare occurrence, and it is unnecessary therefore for us to allude to 

 them any further. It should be mentioned also with respect to 

 the laws of Phyllotaxy, that they are frequently interfered with 

 by accidental causes, which produce coiTcsponding inteiTuptions 

 of growth, so that it is then difficult, or altogether impossible, 

 to discover the regular condition. 



All the above varieties of Phyllotaxis in which the angular 

 divergence is such, that by it we may divide the circumference 

 into an exact number of equal parts, so that the leaves com- 

 pleting the cycles must be necessarily directly over those com- 

 mencing them, are called rectiserial, to distinguish them from 

 those in which the divergence is such, that the circumference 

 cannot be diWded by it into an exact number of equal parts, 

 and thus no leaf can be placed precisely in a straight line over 

 any preceding leaf, but disposed in an infinite curve, and hence 

 called curviserial. The first forms of an-angement are looked 

 upon as the normal ones. The latter will show the impossi- 

 bility of bringing organic forms and arrangements, in all cases, 

 under exact mathematical laws. 



Opposite and Verticiilate Leaves. — We have already ob- 

 served with regard to these forms of arrangement, that the 

 successive pairs, or whorls, as they succeed each other, are not 



