CONTACT-PRESSURES. 249 



converse should be stated — it is the diminution of vitality which 

 renders them degenerate. This tendency to yield to outside 

 pressure becomes, in fact, a measure of the decadence of growth- 

 vitality and constitutes the phenomenon of " packing." Packing 

 thus takes place in the case of both cells and lateral primordia 

 as they attain their adult condition ; and the phenomena ob- 

 served in the packing of cells composing ordinary parenchymatous 

 tissue may be taken as a type of what is to be expected in the 

 analogous case of lateral members. All growing-points lay down 

 cells conceivably endowed at first with equal growth-energy, and 

 arranged in layers the main periclinal and anticlinal construction 

 lines of which, as Sachs pointed out, are probably orthogonal 

 trajectory curves. As the rapid maturation of the specialised 

 peripheral layers involves a reduction in their capacity for main- 

 taining the rate of growth of the cells composing the inner 

 tissues, and these latter tend to round off in order to produce 

 the necessary intercellular spaces, the system falls into the 

 irregular arrangement, approximating hexagonal packing, familiar 

 in tranverse sections of a typical stem or root. Further pressures, 

 especially well seen in the case of thickened members exhibit- 

 ing sliding-growth, produce cell-forms often very approximately 

 hexagonal in section. 



Eesults somewhat similar should therefore be obtained in the 

 case of lateral primordia which develop within a closed space 

 and show feeble growth specialisations. To further satisfy the 

 conditions, an apex is required in which the primordia are pro- 

 duced in a system with a fairly high ratio of curves, and a simple 

 example is afforded by the winter-bud of Cedrus. 



In Oedrus Libani, as in many other Conifers, tlie advanced xerophj'tic 

 specialisation of the perennating foliage-buds takes the form of the 

 protection of the young primordia of the foliage-leaves by means of a 

 ring-growth of the stem which constitutes a well-marked cup, identical, 

 in fact, with the circular zone of growth which in floral shoots 

 represents the first stage in the development of perigynous and 

 epigynous floral structures. Such a ring-growth may be conveniently 

 termed the crater type of apex, as opposed to the normal production 

 of a cone apex. In Gedrus Libani the crater is well marked, and, 

 following the laecbanjcal law of growth for such a lateral structure. 



