Note that all laterals go down to the first year's wood ; the pith of the lateral 

 being continuous with that of the main axis, and laterals diverge at very approximately 

 the Fibonacci angle of 137^°. 



Owing to the laterals being formed in ' false ' whorls each season, the knots are 

 spaced apart at a distance equal to the annual growth of the axis, or commonly 

 1 8 in. to 2 ft. (in Pitch Pine as much as 3 ft. 6 in.). 



Bark : The primary phellogen gives a thin layer of cork cells in radial rows, 

 and up to 50 /a wide, over a period of 7-9 years. The cork tissue differentiated as 

 (i) thin-walled units with wavy walls readily crushed or peeling, and (2) stone-cork 

 layers, 1-2 deep, of units with thick sclerosed and pitted walls. This distinction of 

 hard- and thin-walled zones gives a peeling papery bark, separating readily at the 

 phellogen, and requiring no lenticels. 



At an age of 8-10 years formation of scale-bark commences, by secondary 

 phellogens cutting deeply into the parenchymatous residues of secondary phloem. 

 On older trees a massive scale-bark is produced, the scales 1-2 in. wide, and several 

 long, as layers of red tissue, weathering brown externally. The scale-layers 

 are 1-2 mm. thick, apparently indicating annual periods. The new phellogens 

 give but few units, about a dozen in the radial rows, differentiating as brown thin- 

 walled zones with more colourless thin-walled tissue (50 /a wide), and readily 

 separating. Stone-cells are feebly developed. The whole mass consists mainly of 

 loose dead parenchyma, with no contents and litde cellulose reaction, but a certain 

 amount of lignification. No lenticels are formed, the cracking of the scales 

 apparently affords sufficient aeration. 



Comparison of other species of Pinus : taking P. sylvestris as type of 

 2-needled Pine : — 



(i) P. Laricio (incl. var. ausfn'aca) of S. Europe, with somewhat larger tissues: 

 annual rings of strong growth to 10 mm. wide; pitted tracheides more rounded in 

 transv. sect. In Rad. longit. sect., spring-tracheides 60 /x in radial depth ; 

 bordered pits to 25 /a ; pitted tracheides with sinuous ends, across 3-4 radial rows. 

 M.R.P. with broadly oval pit, i per tracheide ; but also commonly 2 in radial 

 direction : M.R.T. with jagged ingrowths short and blunt. The scale-leaf bases of 

 the main axis persist, and enlarge with the growth of the stem ; the phyllotaxis 

 pattern of scales in contact-parastichies remaining obvious for 6-10 years, much in 

 the manner of the cortex of Lepidodendron ; the rhomboidal areas increasing to 

 20 mm. by 15. As the first thin cork with stone-cell bands follows the pattern, the 

 peeling cork of these areas may give an erroneous impression of scale-bark from the 

 beginning. True scale-bark is only formed later. The persistence of the phyllotaxis 

 pattern is characteristic. 



(2) P, exeelsa, 5-needled Pine; stem with smooth bark for long periods; the 

 first new phellogen cuts deeply into the secondary phloem, delimiting dead tissue, 

 including phloem-parenchyma with calc. oxalate crystals, by thin-walled cork, at 

 20 years old or more; leaving a distinct zone of phloem with layers of crushed 

 sieve-tubes. Naked-eye characters of wood as in P. sylvestris. Rad. Longit. sect, 

 shows : — 



M.R.P. communicating with pitted tracheides by one large rounded pit per 

 tracheide, rarely 2. 



M.R.T. with slightly wavy wall, but no internal jagging; minor ingrowths only 

 in the case of large rays : bordered pits 1 2 ya. 



{3) Pinus palustris (Pitch Pine), 3-needled form, with marked contrast between 

 deep brown summer wood and yellow spring wood ; annual growth considerable in 

 length, but rings narrow, commonly 1-0-5 irim-, pitted tracheides with commonly 

 2-ranked pits in spring wood. 



INI.R.P. typically 2-5 pits per tracheide, in series two deep (range 1-8). 



M.R.T. with jagging so pronounced that the ingrowths meet across the cavity 

 of the cell to a close reticulum : bordered pits small. 



(4) P. insignia {radiata), 3-needled form. Annual rings broad, to 5 mm. 



M.R.P. well defined, crossing as many as 8 tracheides, with 2-5 flared or oval 

 pits per tracheide (rarely i). 



M.R.T. distinctly jagged, with acute teeth ; and mi.xed with M.R.P. in same ray, 

 as many as 4 bands in deeper rays. 



10 



