Gymnosperms : Conifeiae — Pinus — Stem-structure. III. 



PINUS SYLVESTRIS, the most characteristic soft-wooded Conifer of the 

 N. Temp. Tegion, also the most highly specialized type of Gymnospermous tree of N. 

 Europe ; indigenous to Great Britain, but only left wild in Scotland, hence Scots Pine 

 or Scotch Fir. 



I. Spring Shoot. Succulent young shoots 2-3 in. long, cut in May, show 

 the new season's growth in active condition ; the young bifoliar spurs in axils of 

 brown bud-scales, enclosed in silvery membranous scales of the spur-axis, and arranged 

 in spiral construction (5:8); growing-point in T-bud, with investment of scales ; 

 laterals just behind it, to give the ' false whorl ' of the season. 



In transv. sect, about an inch behind the apex, note : — 



(i) Irregular shape of section, 5-8-lobed owing to pressure of spurs. 



(2) Very regular circle of ill-defined V.B. (13-15). 



(3) Cortical region of parenchyma (to 50 y. diam.) with starch-storage. 



(4) Central pith of similar parenchyma (30-50 fi), loosely lacunar. 



(5) Epidermis sclerosed and thickened, stone-cells beneath it at the angles. 



(6) Conspicuous series of resin-ducts in cortex (150-200 /x diam.), with lining layer of secretory 

 epithelium, as small units (15-20 /u) with granular contents. 



The Vascular Bundles are differentiated into protoxylem of small spiral tracheides internally; 

 the beginnings of pitted tracheides of the metaxylem and the first xylem resin-duct may be traced. On 

 the exterior of the V.B. tract parenchymatous protophloem, and radial rows of ill-defined sieve-tubes 

 (10-15 jx diam.) with thick cellulose lining layer. Between x and ^ a broad zone of undifferentiated 

 meristem as the cambium, in radial rows, 12-15 ix wide, and 12-15 cells deep, expressing the activity 

 of bundle growth. Med. rays, of parenchyma elongated radially, are freely produced at intervals of 

 1-5 radial rows. 



Nole, there is no differentiation of endodermis, pericycle, or scl. fibres : leads of 

 vascular strands passing off to spurs are cut obliquely. 



In lofigiL sect., xylem of spiral tracheides only, 15 /a diam., with close spiral 

 thickening. 



In longit. sect, of apex, the dome-shaped growing-point appears as a low curve, 

 •5 mm. across, of very uniformly dividing 15 /x units. The whole apex about 40 cells 

 across, and the rise of young leaf-primordia involving 3-4 cells. 



II. One-year-old Shoots, cut in October, show full season's differentiation of 

 the Vascular Cylinder, as a broad zone of metaxylem surrounded by a narrow zone of 

 Phloem ; cortex with ducts, secondary M.R., and periphery of stem with thin layer of 

 cork beneath persistent epidermis. Foliage-leaves and spurs persistent to 3rd year. 



In Transv. sect, of stem 5 mm. diam., in further detail, from periphery : — 



Epidermis of small cells, sclerosed, with thick cuticularized layer (6 [x). 



Cork, as 2-3 layers, 50 /a or more, of cells with thin wavy walls, also giving the 

 phloroglucin-react. Phellogenhypodermal (30-40/A wide), and 1-2 rows of Phelloderm. 



Cortex becoming loosely lacunar, some cells dying off, others with chloroplasts : 

 resin-ducts now radially compressed. 



Phloem of same characteristic radial rows of sieve-tubes, about 1 5 /a wide, with 

 thick cellulose lining, and granular contents ; radially compressed, and crushed out in 

 older portions ; phloem-parenchyma of larger oval cells, tangentially elongated ; 

 persistent and turgid among crushed sieve-tubes ; ray-parenchyma of similar cells 

 radially elongated. 



Cambium of 2-3 layers of meristem (resting-stage) ; with sharp boundary (by 

 phloroglucin-reaction) against metaxylem tracheides. 



Metaxylem mainly of pitted tracheides, in very exact radial rows, 15-20 /x wide, 

 and about 15 /a radially; as many as 75 in radial series to ^. Bordered pits on 

 radial walls only ; ray-parenchyma in radial series, one cell wide, non-lignified, with 

 starch-content, or in winter months large quantities of fat and no starch. 



Parenchymatous islands, with resin-duct, scattered irregularly as tracts 100 /x 

 wide, of about 20-30 cells, and epithelium of few units (4-8), with crushed and dead 

 adjacent cells : M.R. cells also contain terpenes when fresh. 



Protoxylem of crushed tracheides, and parenchyma, all giving phloroglucin- 

 reaction when dead. 



Medulla cells also dying and giving phloroglucin-reaction ; largest units extend 

 to 100 /x diam. 



7 



