THE AN(;iOSPER.MAH : LEAVES 983 



has repelled some of the ablest investigators, to sav nothing of the great 

 majority of students. 



Before entering into any details, there are three main considerations 

 which should be borne in mind, which may be of help in getting a true 

 perspective. 



Firstly, the symmetry' relationships involved are not peculiar to the 

 arrangement of leaves upon a Higher Plant. Leaves are, as we have tried to 

 emphasize, only a special type of shoot of determinate growth, and the 

 problems involved in their location go far back in evolution, far beyond the 

 evolution of the Higher Plants, indeed beyond the evolution of leaves them- 

 selves, and are inherent in every form of branching which depends upon 

 apical growth. It is only in the simplest Thallophyta, with no apical growth, 

 that branching is irregular. As soon as apical growth became an established 

 habit, regularity and rhythm in branch formation became the rule. 



The axillary position of the branch initials in Higher Plants tends toobscure 

 this truth, for it seems to make the position of the branches dependent on 

 the position of the leaves. Only in the few examples of non-axillary production 

 of shoots, e.g., in the bractless inflorescences of some Compositae, do we 

 see, and see in such cases with peculiar clarity and beautv, that the same 

 rules of phyllotaxis apply to the arrangement of shoots (in these cases, flowers) 

 which apply to the arrangement of foliage leaves. The conclusion that leaf 

 arrangement is only one expression of a fundamental organic symmetry 

 which rules throughout the plant is strengthened, not only bv comparison 

 with the branching of Thallophyta, already referred to, but by the 

 more immediate comparison with the arrangement of vestigial scales, for 

 example on rhizomes, or with the spines of Cacti, which show the same 

 symmetrical order of formation, though remote from foliage leaves in form 

 and function. 



Secondly, that the position of a leaf primordium on the stem apex is 

 determined before even the smallest external indication of its growth is 

 visible. It follows that the determining factors, whatever they may be, are 

 internal and protoplasmic and are those factors which control the distribution 

 of growth-potential in the meristem. External factors, therefore, such as 

 contact-pressures against neighbouring primordia, which only come into 

 play after the rudiment has started to grow, cannot be determining factors 

 with regard to its position, however important they become later in influencing 

 its development. 



There are many reasons for believing that a leaf is closely identified 

 physiologically with the portion of the axis immediately around and below 

 its point of insertion and that a similar demarcation into leaf fields or primor- 

 dium fields exists at the apex. Each leaf primordium, appears in the centre 

 of such a field, and its position is predetermined by the mosaic of fields 

 covering the surface of the growing point. 



Thirdly, that the geometrical relationships which are exhibited by develop- 

 ing primordia when the dome-shaped surface of the apex is projected on to a 

 plane surface, while beautiful and interesting in themselves, are irrelevant 



