984 A TEXTBOOK OF THEORETICAL BOTANY 



to the question of the origin of the primordia. They do not control the initial 

 spacing of the primordia but follow as necessary consequences of that spacing. 

 That this is so, is illustrated by the frequent irregularities or changes of 

 phyllotaxis which may be observed on one plant or even sometimes on one 

 shoot. If the established symmetry had a predetermining effect on the 

 appearance of new rudiments at the apex such changes should not occur, 

 but in fact they do, and they may even be to some extent experimentally 

 controlled, which serves to show that the origination of new primordia is 

 independent of the pattern which develops as an effect of their arrangement. 



A homely parallel may be draw-n with the action of a gardener who plants 

 his successive rows of seedlings equidistantly and alternates the positions of 

 plants in successive rows. He might be interested or amused if it were pointed 

 out to him that by so doing he was creating a quincuncial pattern on the 

 ground, but that has no determining effect upon his choice of spacing, which 

 is dictated solely by the wish to give his plants the maximum of light and air. 

 The causation is, so to speak, physiological, the inevitable consequence is 

 a morphological pattern. 



A similar confusion of cause and effect is illustrated by the contention 

 that leaf arrangement is dictated by the need to avoid the overshadowing of 

 lower leaves by those higher up ; a good example of teleological argument. 

 Its weakness is easily perceived when we consider that some common types 

 of leaf arrangement, e.g., the decussate type (see below) are not efficient in 

 this respect ; or again that the same arrangement may be shown by the scales 

 on a rhizome and the leaves on the aerial shoots ; or lastly that means exist 

 for subsequent adjustment of the positions of growing leaves, whatever their 

 phyllotaxis, to ensure a leaf mosaic. In other words the avoidance of over- 

 crowding among leaves is an effect, not a cause. That it is a desirable effect 

 in most circumstances is undeniable, and it may well be that natural selection 

 has favoured certain types of phyllotaxis in consequence, but that does not 

 invalidate the argument against it as a true cause of phyllotaxis. The study 

 of phyllotaxis has suffered from the historical accident that it was approached 

 at first from the wrong end, that is to say, from the examination of mature 

 shoots with elongated internodes, while the really critical conditions at the 

 apex were not examined until recentlv. 



On mature shoots there are three fundamental types of leaf arrangement : 

 i.e., in opposite pairs, or in circles called whorls, or singly at the nodes. A 

 survey of the families of Angiosperms shows that in certain families one or 

 other of these types may predominate almost exclusively, e.g., opposite pairs 

 of leaves in Labiatae, but it is not possible to say, on the basis of their 

 systematic occurrence, that any one of them appears to be definitely more 

 primitive than the others. Nor is this surprising if we recollect that the 

 establishment of these types must have long antedated the evolution of the 

 Angiosperms. There is some evidence that, in certain families or genera, 

 the arrangement in opposite pairs may have given place to the simple arrange- 

 ment of leaves singly. We have touched upon this previously in speaking of 

 heterophylly, and there is, additionally, the evidence froni seedlings that not 



