VARIATION OF DENSITY IN THE WOOD OF PINUS PATULA GROWN IN KENYA

1. The specific gravity was determined for the individual ringsof twenty-three 13-year-old stems of Pinus patula grown in Kenyaand also separately on the early and late wood of six rings.
2. Significant correlations were found between specific gravityand both ring width and age ofthe ring from the pith. The effectof ring width was slight with more than six rings to the inch.
3. A comparison with Turnbull's data from South African treesofthe same species confirmed his results only in the firstninerings from the pith.
4. The mean specific gravity of theearly wood in five rings was0.368 compared with 0.469 forthelate wood. This high valuefor the early wood and the smalldifference between early andlate wood was due to the thicknessof the walls in the early-woodtracheids. Similar thickwalledearly-wood tracheids were observedin Kenya species of Juniperusand Podocarpus.

     

VARIATION OF DENSITY IN STEMS OF DOUGLAS FIR

In a disk from the stem of a very fast-grown Douglas fir (Pseudotsugataxifolia Brit.), 19 years old, with an almost constant ring-widthof about inch, the density of the individual rings was almostconstant from pith to bark and there was no sign of any increaseoutwards such as was found by Turnbull in fast-grown South Africanpines. Similar results were also obtained with more closelygrown material (mean ring-width about 1 1/0 inch). There appearedto be no general relation between density and ring-width, butthe highest densities occurred in rings less than 6 mm. widein the centre of the fast-grown tree and at the outside of atree with access to river water.     

THE INFLUENCE OF RAYS ON THE SHRINKAGE OF WOOD

Dissections from solid blocks and from microtome sections haveshown that the broad rays of Quercus ilex, Cardwellia sublimis,and Helicia terminalis shrink less radially and more longitudinally(axially) than the intervening tissues. Dried cross-sectionsshow the broad rays projecting from the top and bottom edges,with the other tissues curving inwards. Strips cut with oneside composed of a ray and the other of the intervening tissuescurve on drying, the strip becoming convex on the side of theray. The shrinkage rates of sections vary considerably fromthose of solid wood, but the comparison of one tissue with anothergave similar results whichever method was adopted.     

THE DISTRIBUTION OF MOISTURE IN THE LIVING STEM IN SITKA SPRUCE AND DOUGLAS FIR

1. Equilibrium in the moisture content of the stem appears to bereached at a given percentage of saturation rather than at apercentage based on the dry weight, and this method of expressingmoisture content eliminates many of the fluctuations apparentwith the latter.
2. Summer moisture content in Sitka spruce (Piceasitcbensis, Carr.)was highest on the site with the highestrainfall and lowestwith the lowest rainfall. It decreased inlate summer, suddenlyin Sitka spruce (in August) and graduallyin Douglas fir (Pseudotsugataxifolia Brit.).
3. There was asharp moisture gradient from cambium to heartwood,the saturatedzone being very narrow except on the wet Sitkaspruce site.In the Douglas fir it appeared that water couldbe withdrawnfrom the layers just behind the transpiration streamfasterthan it could be replaced, leaving this zone drier thanthesurrounding layers.
4. Lateral root pruning in Douglas fir ledto the drying out ofa narrow strip of tissue in the stem.

     

Interaction of methyl nitrite with the inorganic and organic fractions of soils

The reactions of methyl nitrite (CH₃ONO), a gaseous product of NO^?₂ decomposition in soils, were studied by exposure of soils in closed vessels to the gas. The N transformations occurring in soils at different soil-water states were assessed by measuring CH₃ONO and other gaseous forms of N in the gas space, soil inorganic N (NH₄⁺, NO^?₂, NO₃^?) and incorporation of CH₃O¹⁵NO into the soil organic N fraction. The initial rate of uptake of CH₃ONO increased with decreasing soil-water content, but the rate of hydrolysis decreased as soil-water content decreased below – 33kPa matric potential. Uptake was not affected by y-irradiation of soils. Adsorption isotherms conformed to the Langmuir equation in each of 22 oven-dry soils studied. Langmuir adsorption maxima were positively correlated with the clay contents of the soils, and adsorption was reversible to some extent at all soil-water states. Small amounts of added CH₃ONO were recovered as N₂ and N₂O and as ¹⁵NH₄⁺ in γ-irradiated soils. From 60 to 72% of added CH₃O¹⁵NO was recovered by Kjeldahl digestion; this was indicative of a chemical reaction with soil organic matter. The results suggest that the physical process of adsorption of CH₃ONO by clay minerals and the chemical fixation of CH₃ONO by soil organic matter are key factors controlling the atmospheric concentration of CH₃ONO, and that the combined effect of these processes, together with hydrolysis in the soil solution, will inhibit the emission of CH₃ONO formed in N-fertilized soils.     

Factors affecting formation of methyl nitrite in soils

The formation of CH₃ONO in 11 soils treated with HNO₂ or NaNO₂ in a closed system, was studied by measuring the concentration in the gas space above the soil and by absorbing CH₃ONO in HI. The gaseous concentration of CH₃ONO increased and then decreased following additions of HNO₂ or NaNO₂, and the production of CH₃ONO increased with increasing concentrations of HNO₂ or NaNO₂ added to soils.
The amounts of CH₃ONO trapped in HI were 13.5 to 20.4 times higher than those determined by integrating under the net production curves. The evolved CH₃ONO amounted to 0.4 to 3.5% of added NO₂⁻, and 4.2 to 50% of the gaseous forms of N absorbed by acidic KMnO₄ solution. The CH₃ONO evolved from soils was positively correlated with the methoxy content of the soils, and inversely related to soil pH, with negligible amounts being evolved from alkaline soils. The results show that CH₃ONO is a product of NO₂⁻ decomposition in soils, and indicate that small concentrations of the gas may be produced in N–fertilized soils in which NO₂⁻ accumulates.