CONTACT -PRESSURES. 247 



On the other hand, evidence that growth ceases to be uniform is 

 seen in the majority of leafy shoots. The quasi-square rhombs 

 become flattened, the system is no longer orthogonal, a peculiar 

 " sliding-growth " usually takes place, and the spirals tend to pass 

 into spirals of Archimedes as the members attain equal volume and 

 are spaced at equal intervals. Such cessation of uniform growth 

 is produced by a lowering of the rate of growth in the lateral 

 members ; and such reduction, if equal in all directions, will tend 

 to loosen the members from their close contact, and the bud 

 " opens out." As the rate of growth is thus lowered in the 

 primordia, contact-pressures necessarily vanish {cf. Opuntia). 

 A special case is, however, general among leafy shoots : the rate or 

 growth diminishes more rapidly in the radial direction than in the 

 tangential, while in the latter the rate of growth may be apparently 

 relatively increased. The same effect would be produced if the 

 radial growth of the axis be diminished at a greater rate than the 

 tangential growth of the leaf, owing to an apparent contraction of 

 the whole system. These phenomena constitute the special case 

 of the hilaterality of the so-called dorsiventral leaf, and may be 

 considered separately. It is so far clear, however, that the effect 

 of an increased tangential growth, real or apparent, must induce 

 sliding of the members over one another ; but it does not follow 

 that an internal thrust on the part of the members themselves 

 can ever convert the system into any approximation to the 

 hexagonal packing of the "pile of shot" type.* 



* That is to say, if an orthogonal system of vertical and horizontal rows of 

 bodies, free to roll over each other, be acted on by an external horizontal force 

 the horizontal rows are retained, but the vertical ones are displaced so as to 

 intersect the horizontal at 60°. In the circular system of a transverse section, 

 the vertical rows are represented by radii, and the horizontal by the circular 

 paths : in the corresponding asymmetrical case, the vertical rows may therefore 

 be represented by the shorter curves, the horizontal by the longer ones. The 

 general result of any lateral thrust on the part of the members themselves 

 will be that the shorter contact-curves become broken ; and this again is the 

 phenomenon usually observed as soon as the primordia become markedly 

 bilateral ; while the longer curves are retained imaffected, and are thus rendered 

 increasingly conspicuous. On the other hand, a vertical compressing force 

 (Schwendener), acting along the shorter curves therefore, would have 

 produced similar flattening appearances, but would have tended to maintain 



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