CONSTANT PHYLLOTAXIS. 99 



Phyllotaxis of Aeaucaria EXCELSA. 



Arauearia ecccelsa, K. Br. — A good example of this relation of the 

 " bulk-ratio " to phyllotaxis phenomena is afforded by such a plant- 

 form as Arauearia excelsa, in which, owing to the remarkably small 

 development of the foliage-leaves, and the absence of special 

 growth-modifications in them, the symmetrical development of 

 the plant is allowed free scope. 



The main axis of a young plant grows erect and produces leaf- 

 members in well-defined (8 + 13) system, the primordia being 

 pressed into uniform rhomboidal areas, and the transverse section 

 shows a very close approximation to the theoretical vortex-con- 

 struction. 



The secondary axes are spaced symmetrically round the main 

 stem and hence follow the Fibonacci series, 5 or 8 being the most 

 usual. When growth is vigorous these axes similarly present the 

 (84-13) system but develop horizontally (figs. 40, 41). 



Tertiary axes are produced in two rows only, along the flanks of 

 the secondary axes, which may be so far regarded as presenting a 

 certain dorsiventrality in the space-form of the branches. Section 

 of the apex shows that the spiral construction is utilised and not 

 disturbed, the lateral branches of this order being arranged along 

 two lateral " orthostichy " lines (Phyllody spirals) towards the upper 

 surface, the spiral character of which may be observed on the 

 plant. It will be observed that these lateral shoots arise in the 

 axils of two successive members of each full cycle {e.g., 12, 13, and 

 33, 34, in fig. 40). 



These tertiary axes produce leaves in the system (5-|-8) (fig. 41), 

 and as a general rule develop no further. But when a leading bud 

 of a secondary axis is damaged, the end branch may assume its 

 place and similarly produce lateral shoots in two rows. An example 

 of such a bud whose construction showed the (5 -|- 8) system gave 

 rise to a large number of laterals, all of which were constant and 

 homodromous at (34-5) (fig. 41). 



Apices of lateral branches of the first degree may also be found 

 on the same plant exhibiting the anomalous ratio (74-11) (fig. 41). 



