468 DR. P. GROOM AND MR. W. RUSHTON ON THE STRUCTURE 
this sub-section. Tt agrees with typical members of the Haploxylon Section 
in its uniseriate pits on the radial walls of the wood-tracheids, 
In altitudinal range there is not much difference between P. excelsa 
(6,000-12,000 ft.) and P. Gerardiana (5,000—10,000 ft.), but the latter alone 
is xerophilous in habitat: and its leaf has a much thicker cuticle and the 
hypoderma has thicker walls. The greater development of transfusion-tissue 
in P. Gerardiana perhaps might be associated with the greater thickness of 
the leaf, but Max Scheit (1883) concluded that the amount of transfusion- 
tissue varies directly with the external influences promoting transpiration, 
and the great development of the resin-ducts in this specles recalls the 
extensive production of ethereal oil glands in the leaves of plants in dry hot 
places, 
Pinus longifolia, P. Khasya, and P. Merkusii. 
These three species agree not only in the diploxvlie nature of the leaves, 
the persistent nature of the sheath of the dwarf-shoot, and the possession of 
a central umbo on the thick cone-seale, but also in that the transition from 
spring-wood to summer-wood is sudden (except in P. Ahasya), the outermost 
tracheids of the annual ring do not universally bear pits on the tangential 
walls, the pits on the radial walls of the spring-tracheids are often 2-seriate, 
and the ray-tracheids are denticulate. 
As already pointed out, these three species that frequently grow under 
subtropical or even tropical conditions are, like the sub-tropical American 
pines, the species possessing the widest spring-tracheids, despite of the fact 
that P. longifolia, and apparently sometimes the other species, live in situa- 
tions that are at least temporarily dry. As regards leaf-structure all three 
have stomata on all their faces, P. longifolia, the most clearly tending to 
xerophily, has the thickest cuticle (though not so thick as P. Gerardiana), 
and P. Ahasya has the thinnest. P. Merkusii has the most numerous lines 
of stomata, as its twelve lines of stomata imply that these are about *2 mm. 
apart on the average ; whereas in 7. Nhasya and P. longifolia, with 10 and 
8 stomatic lines respectively, these are on the average *26 mm. apari. In 
the more xerophilous P. longifolia the endadermis has markedly thiekened 
outer walls, in P. Mer£usii this is less marked, while in P. Ahasya there 
is no indication of such thickening. All three species contrast with the 
haploxylie Indian species in the feebler development of the tissue separating 
the endodermis from the vascular tissue. The sub-endodermal layer forms 
a fenestrated cylinder of transfusion-tissue, in addition to this there is usually 
only one layer of complementary tissue, excepting in P. longifolia where 
there is a disproportionate development of mechanical tissues 1-3 cells thick. 
The xylem is well developed in all three species. The two three-needled 
species have merely two marginal resin-ducts, whereas P. Merkusii also has 
a median one against the outer face : in all three cases the ducts are against 
