38 BOTANICAL GAZETTE [january 



uneven and covered with the free cones of the young adults, while 

 spaces creep up from below. These result because lateral growth is 

 insufficient to maintain closure. Increased lateral spacing now 

 permits increased lateral growth, one of the prime factors slowing 

 vertical elongation. Approaching the adult stage (age 4) the layer 

 breaks up into its component tree masses. This occurs by rifting 

 (vertical or horizontal breaks due to tree or branch fall), the gaps 

 becoming nearly unfillable at age 3, for closure is either by elonga- 

 tion of a younger tree or by lateral growth of the adjacent tree 

 circle. This age is the first one free vertically and laterally. 



A further step is the breaking up of a tree unit into foliage 

 clumps, one or several to a branch. Finally, many of the oldest lose 



* 



all primary foliage, the trunk and branches bearing scattered hand- 

 fuls of leaves. This secondary foliage is borne on slender twigs 

 developed from adventitious buds. Gradual fall of the last age 

 destroys all semblance of a foliage stratum. 



Recession occurs in two main ways (trunk elongation unimpor- 

 tant) : by shedding of leaves and branches at the synfolium base (the 

 synfolium is self -pruning during the growing season) , and by apical 

 growth, the stems adding new leaves and branches, thus extending 

 the synfolium compass vertically. With increase of synfoliar dis- 

 tance (from ground) and rifting, the herbage layer receives increas- 

 ingly stronger light; thus the tree seedlings are stimulated to more 

 active growth and the illumination of the forest floor decreases again. 



The sapling synfolium contrasts with the trunk strata above 

 and below, in apparent space occupied, color, and opacity. The 

 lighting of the trunk stratum above is much greater, and that 

 of the dead branch layer much less, being composed of flat, thin, 

 horizontal tissue plates. The synfolium seems to have the ideal 

 structure and arrangement for maximum of surface, light absorp- 

 tion, synthetic efficiency, and carbon dioxide use, together with the 

 minimum material, volume occupation, and transpiration. The 

 apparent effect on the eye gives impressive display and exaggerated 

 idea of solidly filled space. This effect is heightened on passing 

 from the bright sunlight into the dense shade of the forest. 



Yapp (26) makes some interesting observations on evaporation 

 at different levels in an English marsh, and Sherff (22) on an 



