The Formation of Red Wood in Conifers. 
BY 
ALFRED J. EWART, D.Sc., Ph.D., F.L.S. 
Professor of Botany in Melbourne University . 
AND 
ARCHIBALD J. MASON-JONES. 
University of Birmingham. 
With Plate XV. 
T HE formation of red wood on the under sides of the branches of 
Conifers, which was studied in detail by Hartig, has been variously 
explained as being due to gravitational stimuli, to variations of pressure 
and tension, and to differences of illumination on the upper and under 
sides h 
The latter explanation has in part been adopted by Sonntag 2 , who 
has recently investigated and compared the mechanical properties of the 
red and white wood of Conifers. 
It is not easy to see what direct effect differences of illumination on 
the two sides can exert upon the cambium of old stems covered with thick 
opaque layers of bark. 
The only possible action in such cases would be due to the fact that 
the exposed upper side might be slightly warmer during the day time than 
the shaded under side. 
In the case of Pinus , owing to the height of the branches above the 
ground, the effect of nightly radiation on the relative temperature of the 
upper and under sides is negligible ; and in that of Cupressus , since mainly 
lower branches of compact trees are used, the under side would be more 
exposed and subject to greater radiation and cooling during the night 
Hence, the only possible effective differences of temperature are those 
produced during the day time. 
Sonntag lays main stress upon the influence of pressure, which he 
considers to be primarily responsible for the production of red wood, the 
1 Cf. Pfeffer’s Physiology, Eng. Trans., Vol. ii, p. 108 ; Vol. iii, p. 416. 
2 Sonntag, Jahrb. f. wiss. Bot. Bd. xxxix, p. 71. 
[Annals ot Botany, Vol. XX. No. LXXVIII. April, 1906.] 
