102 RELATION OF PHYLLOTAXIS TO MECHANICAL LAWS. 



On drawing sections of such a series of buds under the same lens, 

 with camera lucida, it becomes clear that the leaf-members in all 

 are practically identical, and the leaf-primordium a fairly constant 

 quantity, while the axis is variable. 



From the previous table of the curve-systems — 



(8-1-13) corresponds to a bulk-ratio of 5 : 1, 

 (5-f- 8) „ „ „ 3 : 1, 



(3+ 5) „ „ „ 2 : 1, 



respectively ; while by drawing a special figure for the system 

 (7-1-11) it may be shown that the bulk-ratio in such case is very 

 approximately 4 : 1 (or 4'2 : 1). 



From this standpoint, therefore, Araucaria excelsa builds its 

 phyllotaxis system according to the relative size of the axis 

 concerned, the ratio being expressed with remarkable accuracy to 

 the nearest whole number, according as the diameter of the axis is 

 5-, 4-, 3-, or 2- times as large as that of the primordium to be 

 placed on it. 



The occurrence of the ratio (7-1-11) is of especial interest in 

 that it is associated with the normal series, not so much as an 

 exceptional member of another hypothetical series, but definitely 

 as the ratio which gives the missing intermediate bulk-ratio of 4 : 1. 



Again, comparison of these diagrams shows a striking analogy 

 between the number of primordia of fairly constant bulk placed on 

 axes of different sizes and the segmentation of the tissues of such 

 growing points into constituent cells which also have an approxi- 

 mately equal volume in all. In fact, just as a growing point which 

 is well nourished contains a large number of cell units, and an im- 

 poverished one only a relatively small number, so one is bound to 

 conclude, the number of primordia arranged on an apex is an 

 expression of the strength of the growing point. 



Whether dealing with cell-segments or lateral primordia, a plant- 

 apex, to use a common metaphor, must cut its coat according to its 

 cloth, and so long as the lateral members remain approximately 

 constant in volume, so the number must vary. In correlation with 

 the relative bulk-ratio of axis and member, a certain number of 

 curves are therefore selected. 



