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RELATIONSHIPS. 



stem-surface of bark. Tims the armor and outer cortex are continually removed 

 from the base of the trunk towards the apex, with very greatly varying rapidity in 

 the different genera and species. Thus it is, too, that the periderm, and such bark 

 as it may give rise to, comes to be the final outer covering of the trunk from its 

 base, where the peripheral cortex may be cut into, all the way to the leaf crown, just 

 beneath which it ends in the tips of old leaf bases of the preceding foliar series. 

 Upon the rate and regularity of periderm and bark formation (or conversely 

 the vitality of the old foliar bases) must then depend the degree of armor retention. 

 This explains how the cycads come to present every variation from the heavily 

 armored species of Cyras and Encephalartos to the naked or tuberous Zamias and 



Fig. 1 17,— Cycas media. X £. A small plant, showing rapid excision of the 

 armor, whence it appears that persistence of the armor is not necessary to 

 increase in trunk height, this species being the tallest known. (Compare 

 with figure 102.1 



Bowenias. In the latter genera there is a very regular and clean excision of the 

 armor, and the cortex of all the lower portions of the trunk ends in the thin and 

 even layer of periderm. In the case of an old trunk of Macrozamia Fraseri there 

 is a very different appearance. In this cycad, according to Worsdell (205), the 

 periderm is recognizable to the eye at the outer limit of the cortex as a conspicuous 

 narrow white zone with an extremely irregular sinuous course, due to the fact that 

 the successive peridermal layers arise in a very irregular manner and often in the 

 phelloderm of the next older layer. As a result, the outer surface of the stem, 

 consisting in either the remains of leaf bases or portions of cortex in which peri- 

 derm excision has begun, is left very rough and jagged. The cellular make-up of 

 the periderm is distinct, the cork layer being quite uniformly composed of crushed 

 cells and the inner phelloderm layer of thinner-walled cells ranged radially with 

 the cork and having interspersed various thicker-walled cells and idioblasts. 



