Gymnosperms : Conifeiae Pinus Stem-structure. III. 



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

 N. Temp, region, 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 : 



(1) 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 p diam.) with starch-storage. 



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



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



(6) Conspicuous series of resin-ducts in cortex (150-200 p 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 V- diam.) with thick cellulose lining layer. Between x an d <f> a broad zone of undifferentiated 

 meristem as the cambium, in radial rows, 12-15 A 1 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. 



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

 vascular strands passing off to spurs are cut obliquely. 



In longit. sect., xylem of spiral tracheides only, 15 ft 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 fx 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 ft). 



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

 phloroglucin-react. Phellogenhypodermal (30-40 \x 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 15 ft 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 fx wide, 

 and about 15 ft 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 ft 

 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 fx diam. 



7 



