VARYING GROWTH IN LATERAL MEMBERS. 319 



and constitute a "hud" so that the formation of a terminal 

 bud-cluster of members and its subsequent expansion thus becomes 

 a question of the correlated sequence of these growth-variations. 

 While, then, the general effect of elongation in the main axis 

 will be to diminish the number of members seen in transverse 

 section, this may be compensated to a certain extent by extensive 

 growth in length on the part of the lateral members themselves. 

 Simultaneously, the approach of adult members to a uniform bulk 

 results in the secondary production of an effect of spirals of 

 Archimedes, as already noted. 



Hence, the best phyllotaxis patterns will persist in shoots whose 

 leaves undergo little special modification, and remain either in a 

 primitive condition as protuberances of rhomboidal section, or are 

 elongated to needle-like members of similar rhomboidal section.* 

 Good examples are therefore afforded by Araucaria excelsa (fig. 

 41), Cryptomeria japonica, Pinus Pinea (primary leaves) (fig. 

 88), Cedrus atlanticus (primary leaves). 



Araucaria excelsa (figs. 41, 42). — The leaves retain the rhom- 

 boidal section of the " square " areas with considerable accuracy, 

 the orthogonal intersections are fairly well marked, and the 

 solitary vascular bundles of each member are formed very obvi- 

 ously at the " centre of construction " (most clearly observed in 

 fig. 42 (7-1-11)). 



Similar phenomena are even better marked in the seedling of 

 Pinus Pinea (fig. 88); the orthogonal intersections are in some 

 cases closely approximated toward the centre of the system, and 

 so clearly is the original construction retained that the disturbing 

 factors are readily isolated. These are seen to consist of : (1) the 

 diminution of tangential growth of members as they reach the 

 periphery of the plane of section, so that each member of a spiral 

 path subtends a smaller angle than its successor, except in the 



* A source of error is also introduced owing to the fact that the primordia, 

 however perfect in themselves, are inserted on a curved base, and do not, 

 therefore, extend in a vertical direction. A longitudinal section is therefore 

 necessary in order to see which primordia are sufficiently accurately placed 

 to be taken as typical. This error tends to be reduced as the apex increases 

 in diameter ; hence the value of Composite capitula (Helianthus) as a type. 



