Gymnosperms : Pinus ; Morphology and Reproduction. VI. 



PINUS SYLVESTRIS, Scots Pine, Scotch Fir; Forest-tree of N. Europe 

 and Asia, not native in N. America ; the only indigenous species of Pinoid, and the 

 type of the group; 100-120 ft. high, and 3-4 ft. diam. ; evergreen, pyramidal when 

 young; leader erected, laterals horizontal, with end-shoots ascending at about 45 ; 

 ultimately fiat-topped by shading of lower branches and cessation of growth in height. 



Shoot-system with foliage comprises 5 types of axis and 10 of leaf-member, as 

 the expression of considerable specialization in shoot-construction: (1) Cotyledons, 

 (2) Juvenile leaves, (3) Bud-scales, (4) Scales of main axis, (5) Scales of spur-base, 

 (6) Foliage-needles, (7) Bracts subtending staminate flowers, (8) Scales at base of 

 floral shoots (perianth), (9) Stamens (microsporophylls), (10) Bracts of the ovulate 

 cone. Axes, (1) Erected leader, (2) Laterals, (3) Foliage-spurs, (4) Staminate flowers, 

 (5) Ovulate flowers. 



Annual Shoot : Scale-leaves in spiral phyllotaxis, (5 : 8) in section of bud-apex 

 T-bud pointed (7 mm.), with many bud-scales and conoidal apex ; season's growth 

 6-18 in. (reduced to 1 | in. only in starved laterals). Lateral leaders formed only 

 in axils of few (1-5) scales at the distal end of the shoots, giving ' false whorls ' as the 

 expression of annual increments of main axis ; none on starved laterals. 



Spurs in axils of all remaining scales, with basal membranous sheathing leaves 

 and suppressed apex ; each bifoliar, with 2 foliage-needles, 2-| in., to 1 in. on starved 

 laterals, fiat upper surfaces superposed, functional 2-3 (5) years. 



Note extreme xeromorphic specialization of 'needle '-habit, leaf-anatomy, absence 

 of hairs, terpene-exudation as heat-screen, effect of stiff needles in reducing wind- 

 velocity, sunken stomata, reduced leaf-area, feeble vascular supply. 



Flowers, diclinous (monoecious), spirally constructed, apetalous, anemophilous; 

 available for a week or so at end of May. 



I. Staminate Flower: borne in considerable numbers (100) in spicate 'inflor- 

 escence ' aggregation, in axils of scale-leaves at the base of new spring-shoots, and 

 replacing foliage-spurs : prepared in winter-bud, maturing in April, functional before 

 the new foliage is grown (May 25). Basal scales constitute a rudimentary perianth ; 

 stamens 60 or so, in spiral series, 2 mm. long ; filament negligible, with overlapping 

 connective-crest: pollen-sacs (2) on lower surface, each dehiscing longitudinally by 

 mechanism of banded epidermal cells. Pollen grains are formed in tetrads after meiosis. 



The floral axis elongates in expanded flower, 3 mm. to 5-10 mm. : Pollen yellow, 

 oval, 55 /a long, with 2 inflated 'wings', 80 jx over all : output enormous, cf. ' sulphur 

 showers' ; rough esiimate a litre per tree : old flowers withered are soon shed, leaving 

 bare places on annual shoot axis. 



II. Ovulate Flowers, as young ' cone '-formations, of obscure morphological 

 value. The term carpel may be avoided, as the relation of the paired ovules to the 

 primary leaf-units is not apparent at first sight. The term flower will be used 

 provisionally as implying an axis comparable with the staminate shoot. This ovulate 

 construction is found replacing leader-shoots (1-3) only at the distal end of the 

 annual growth. Similarly performed in the winter-bud (1 mm. Dec.\ emerging with 

 the foliage-needles in May, and erected, 5 mm., surpassing the T-bud. 



Constructed of rose-coloured, spirally-arranged, close-set ' bract-scales ', grading 

 from basal rudimentary ' perianth '-members to functional ' bracts' with 'cone-scale ' 

 growth in axil ; the latter associated with 2 ' inverted ' ovules in the angle on the 

 upper surface, diverging obliquely outwards. 



Note. The ' bract-scale ' probably expresses a vestigial carpel ; the ovules being 

 now relegated to the axillary region ; the ' cone-scale ' is a secondary growth originally 

 used to seal the primitive cone, now precociously formed and functional at pollination. 



Floral Mechanism : intercalary extension of the cone-axis allows separation 

 of the bract-scales by narrow chinks, so that wind-borne pollen falls between them 

 (May 20-25). The ovules (-5 mm. long) consisting of nucellus, one integument, and 

 micropyle, represent the megasporangia ; the megaspore at this stage being a single 

 cell centred at the level of the integument-base. The nucellus secretes a drop of 

 fluid, and pollen-grains drifted down the chinks are withdrawn through the micropyle 

 (drop-mechanism), to anchor on the nucellus-apex by a short germ-tube. 



The Green Cone : The pollen-grains are shut in by a growth of the integu- 



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