(3) Alhumen-celh, a group of 6-12 small cells (15-20 /^) with densely 

 granular contents and large nuclei, on the outer flanks of the phloem 

 of each bundle. 

 (yS) Vascular Bundles, 2, obliquely orientated, with xylem towards upper 

 flat surface, of — 

 (i) Xylem of spiral tracheides (protoxylem), few, followed by pitted 

 tracheides (10-15 p) in radial rows (4-5 deep); interrupted by radial 

 rows of xylem (ray) parenchyma, at intervals of 1-5 tracheids. 



(2) Phloem of similar radial rows of sieve-tubes (6-8), interrupted by 

 continuation of ray-parenchyma (often with starch-grains). 



(3) Delimiting these, 1-2 rows of cells may be classed as 'cambium', as 

 the bundles show slight increase in the 2nd and 3rd A-ears ; the radial 

 rows of xylem increasing to 6-8, and those of phloem to 12-20. 



(y) Between the bundles, and especially over the phloem regions, a band 

 (more or less x-shaped) of ' Central fibres ' (30 yu.) with thick walls and 

 tannin contents, wall 10-15 }x thick, with slit-pits more or less sclerosed, 

 particularly characteristic oi P. sylvestris \ some very large (50 /a). 



In longitudinal section note much pitted and elongated walls of epidermis, 

 guard-cells (60 fx) ; mesophyll without plication, in palisade 2-3 layers deep ; free 

 intercellular spaces; endodermal cells 150-200// long, much pitted. Transfusion- 

 tracheides 50-150 /a long. 



In tangential section of surface, note stomatal lines, conspicuous guard-cells, 

 lignified, 60 /x by 45 over all ; elongated slit 30 /x by 6 ; aperture of outer chamber. 



Range of Type ; finest needles on main leaders may run 4 in. long and 2 mm. 

 broad, with over 20 stomatal lines (26), and 16 ducts less peripheral. The stele may 

 show 1-2 small accessory central V.B., these have no albumen-cells. 



Starved leaves of lower laterals, with annual shoot reduced to 13-8-3 bifoliar 

 spurs, with staminate flowers and growth of r inch or less, may be 20 mm. long, less 

 than I mm. wide, with 4 stomatal lines on upper surface, and 4 ducts. 



Xerophytic Factors. The clue to all peculiarities is to be traced in extensive 

 xerophytic adaptations, as the response to (i) violent winds, (2) intense insolation, 

 (3) extremes of temperature. As opposed to general conception of a mesophytic leaf- 

 lamina, horizontally extended to give maximum surface for maximum illumination, 

 note: — (i) elimination of bilateral lamina, (2) elimination of orientating petiole, (3) 

 elimination of horizontal position, (4) ehniination of outward dorsiventrality. The 

 structure is restricted to an intercalary basal growth in a scale-leaf, of petiolar nature, 

 including the first bifurcation of the single-trace bundle (cf. Ginkgo-\)Q\\o\Q); and 

 irregularities affecting 2 primordia together may give a ' double-needle ' (cf. Scia- 

 dopitys). Intercellular spaces, surface exposed, vascular connexions and tracheides 

 are minimized. As new departures, note, sclerosis of epidermis and hypoderm, 

 screen-function of fibres, sunk stomata, more rigid lignified guard-cells, transfusion 

 tracheides as connected with the xylem (particularly well-developed in Pinus among 

 other Conifers. The size of the leaf becomes a general guide to the amount of 

 photosynthetic work done, and is correlated with the amount of polysaccharide avail- 

 able for cone-building : hence intense insolation or a larger needle implies larger and 

 more massive cones on the annual shoot. 



Cotyledons (6-8), about 20 mm. long, and -6 mm. broad, triangular in section, 

 with very generalized construction ; no sclerosed layers, ducts or endodermis ; 

 stomata on two radial faces only, plicate mesophyll rudimentary; one central bundle, 

 traces only of transfusion-tracheides. 



Juvenile Needles of first year, very similar ; rhomboidal in section, becoming 

 D.V. ; with siornata on both sides ; one median bundle, 2 ducts ; endodermis and 

 transfusion tracl)eides distinct; mesophyll more definitely plicate. 



Winter-buds of bud-scales formed from primary leaves of entire annual shoot 

 (to 100), 5-10 mm. long; conspicuously large in the staminate inflorescence : bases 

 succulent and containing chlorophyll ; distal portion of dead tissues with thickened 

 and lignified epidermis ; tips and edges reduced to a lamella of one layer of elongated 

 cells, the marginal region frayed out and looking like hairs. 



Note. New needles continue growth throughout the season, and may elongate 

 slightly in the second year. Spurs are d( tached by an absciss-layer. 



13 



