458 DR. P. GROOM AND MR. W. RUSHTON ON THE STRUCTURE 
5. Prsvs MERKUSH, Jungh. & de Vriese. This two-needled species is tropical 
in distribution. Occurring in Burma and the Malay Peninsula at altitudes 
varying from 300 to 3,500 feet, as well as in Sumatra and elsewhere. 
Apparently it can grow in dry or moist places, occurring, for instance, in. 
very hot dry forests in the Shan Hills (according to H. N. Thompson). 
A number of problems present themselves in regard to the structure of 
the wood of these species. 
[n the first place, what light is shed on their affinities? Or, as regards 
habitat, do the species growing under tropical or xerophilous conditions show 
any corresponding structural features in their wood? and are these 
correlated with the histological structure of the leaves? Incidentally we 
found ourselves faced with the problem as to the nature of ** Sanio’s bars ” 
and markings on the radial walls of the wood-tracheids. 
This paper is divided into two parts, the first of which gives the general 
conciusions and summary of results, the second part contains the special 
description of the different species. 
GENERAL CONCLUSIONS. 
The structure of the wood of all five species conforms with that of typical 
species of Pinus in the following features. There is a heart-wood. Annual 
rings are distinet, as the spring-tracheids have wider lumina and thinner walls 
than the more compressed summer- (“autumn "-) tracheids, The radial walls 
of the tracheids bear bordered pits, which are smaller and scantier in the 
summer-wood and have narrower orifices. There are longitudinal resin- 
ducts, the epithelium of which is thin-walled and develops thyloses. The 
medullary rays are of two kinds : uniseriate, which are one cell thick, and 
pluriseriate, which are several cells in thickness but taper towards the upper 
and lower margins, where they are one cell thick, so that in tangential 
section these rays are fusiform. These latter, so-called fusiform rays, include 
a resin-duct. Some of the medullary rays of all species have ray-tracheids, 
or parenchyma or both. 
(a) Annual Ring. 
The species Pinus longifolia shows double annual rings (РІ. 94. fie. 18). 
I ge g 
o 
[>] 
(b) Size of Tracheids. 
The tracheids are shortest in the xerophilous Pinus Gerardiana, 3 mm., 
altain a length of 4 mm. in P. excelsa. and P. longifolia, and 4°6 mm. in 
P. Khasya, and the relatively great length of 7 mm. in the tropical 
P. Merkusii. This last species also has by far the widest spring-tracheids, 
whose radial and tangential diameters are about 68:5 and 45 u respectively 
