CONIFERAE: THE SCOTS PINE 



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But the thickening encroaches upon the area of th< pi< and overan 1 • 

 418, C, D, E). As seen in surface \nw a double outline then appears. "1 1 1 « - 

 outer circle corresponds to the area ol tin- pit mem b rane, tin mri' 1 to th< hunt 

 of the overarching ; and the greater the thickening the further these outlii 



will be apart. Meanwhile the centre of the pit-membrane itseli tin. k< 

 forming the " torus " (A, C), which serves mechanically to meet th<- n>k ol 

 rupture following on any unequal pressure on the t\v<. Bides. For the torus 

 would, as the membrane yields, press against the overarching lip (li). '1 he 

 prevalence of bordered pits indicates that they are functionally important. 

 They may be recognised as a compromise between the conflicting requirements 

 of ready transit of liquid between thick-walled cells and the maintenance of 

 mechanical strength. For the former a large pit-membrane is an advanta 



B. 



Fig. 418. 



Bordered pits of tracheides of Pine. A = a whole tracheide in transverse section 

 with pit sin its radial walls. B shows the torus pressed to one side. C, I), E illustrate 

 the development, and the relation of the structure as seen in section to the double 

 outline as seen in surface view. £ shows this in the young state. D rather more 

 advanced. C, mature. 



but it would weaken the wall. This difficulty is met by the overarching 

 as seen in the bordered pit, by which the strength of the woody wall is 

 maintained. 



The phloem consists chiefly of sieve-tubes (v, Fig. 419), which are also arranged 

 in regular radial rows, but without companion-cells. They have cellulose walls 

 and sparing contents. Cells of phloem-parenchyma are also present. The 

 sieve-pits (vt) are mostly on the radial walls, and thus correspond in position 

 to the bordered pits in the tracheides. The secondary tissues are traversed In- 

 medullary rays, as in Dicotyledons (em, sm, tm). They include cells that 

 retain their protoplasm, while minute intercellular spaces pass radially inwards 

 between their cells. They serve accordingly for radial ventilation, as well 

 as for storage within easy reach of the conducting phloem. Though the plan 

 of construction of the vascular tissues of Gymnosperms is the same as in 

 Dicotyledons, the details of their development are not so elaborate 



The chief comparative interest of such a plant as the Scots Pine 

 lies not so much in the form and structure of the sporophyte-plant, 

 as in the details of its propagation. This is carried out, as in t he Ailgio- 

 sperms, by organs grouped as Flowers, which are " male " or "female" 

 In the Scots Pine these may be borne on the same tree, though often 

 en distinct branches. The female flower, pink and succulent at 



