Gymnospermae Coniferae Pinoideae Abietineae Pinus. V. 



The Foliage Leaf {Pinus sylvestris as type) ; a remarkable construction with 

 extreme xerophytic specialization as the characteristic Pine-' Needle ' ; a slender, 

 rigid leaf-lamina, about 2 in. long, and i*6 mm. broad (reducing to 1 in. long, and 

 less than 1 mm. wide on starved laterals), with approximately semicircular section, 

 since 2 closely adpressed units fill a cylindrical bud within a sheathing investment of 

 membranous scales of the bifoliar spur : Normally persisting for 2-3 years. 



Note that two units, face to face over the spur-axis, represent a minimum 

 condition of spur-organization for D.V. leaves (the exceptional case of one leaf gives 

 an asymmetrical construction) and older types extend to 3-5 needles on the spur, 

 as a contact-cycle of spirally arranged leaf-members. The lamina may be regarded 

 as based on a pointed scale-member, in which active intercalary growth gives a longi- 

 tudinal extension which may be practically indefinite, without corresponding lateral 

 extension ; i. e. the more this growth-phase has been exaggerated the longer the 

 ' needle '. The edge is minutely serrate, with small teeth formed from exaggerated 

 'angle-cells', spaced '23 mm. apart ; the lamina often shows a slight spiral twist, 

 and the stomata are arranged in conspicuous stomatal lines (marked by wax), 

 usually io-n on the upper face, and 12-13 on the convex lower face : stomata are 

 formed in these rows at 10-12 per mm., and a total estimate for a 2-inch needle 

 gives 12-14,000. The general structure is most readily followed in transverse 

 section of the middle portion of the needle: Reagents, Iodine-sol.; Phloroglucin- 

 H 2 S0 4 : Eau de Javelle. 



I. Epidermis, a uniform peripheral layer of cells, isodiametric (20 p), with 

 cuticularized layers and cuticle on outer face, cavities reduced to small dot-lumina, 

 walls ultimately more or less sclerosed, the thickening showing 4 diagonal slit-pits at 

 angles. Stomata for each stomatal line, with conspicuous guard-cells, owing to broad 

 sclerosed zone in each, and starch-content, sunk 30 /a below surface ; outer chamber 

 flask-shaped with narrow external aperture (10 p) ; subsidiary-cells with thick outer 

 wall build the outer chamber. Exaggerated growths of epidermal cells of angles give 

 the serration of the edges. Hair-developments wholly wanting. 



II. Mesophyll, much differentiated and including resin-ducts. 



(a) Hypodenn, of sclerosed fibrous cells, 1 layer, interrupted at the stomata, 

 2-3 layered at angles of leaf; much less thickened than epidermal cells, 

 but more lignified ; small slit-pits only. 



(ft) Plicate Mesophyll of 3 (4) layers of large cells (50-90 /*), fairly isodia- 

 metric, with pocket-evanginations (3-6), as the expression of the transverse 

 growth of cells to fill all intercellular spaces : Nucleated, with closely 

 packed lining-layer of chloroplasts (3 /x) and included starch. The double 

 nature of the peg-like folds (15 /*) is seen on clearing with Eau de Javelle. 

 The plications along the surface are in the manner of a palisade-series. 

 A single U-shaped cell blocks the cavity below each stoma. 



(y) Resin-ducts, peripherally placed, in contact with hypoderm ; typically one 

 at each angle, 1 (2) on the flat side, and 3 (4) on curved side ; total 6-9 : 

 ducts about 75 //, diam., with delicate secretory epithelium (r layer), and 

 peripheral zone of thick-walled fibres (30-50 fx) with slit-pits, small lumen ; 

 the thickening shows little lignification even in the oldest leaves. 



(8) Endodermis, of large oval cells (40-50 fj), without chloroplasts ; radial dot 

 replaced by extensive radial lignification : contents may include a few 

 starch-grains. 



III. ' Stele ' (a convenient term, following the usage of other stem-terminology, 

 for tissues within an endodermis ; as also ' pericycle ', V.B. with ' med.-rays '). 



(a) Pericyclic region of : 



(1) Parenchematous cells, oval, 50-60 /*, turgid, with protoplasmic contents 

 and starch, arranged perpendicularly to endodermis, and 1-2 deep, alter- 

 nating often very regularly with 



(2) Transfusion-tracheid.es, with thin walls, deformed by pressure of 

 preceding, with numerous bordered-pits (5-6 fx) on all sides ; in series 

 of 2-3, more or less radiating from flanks of xylem, but evenly distributed 

 around the whole contour of the stele. 



II A3 



