306 



BOTANY OF THE LIVING PLANT 



radial rows, not disturbed as the wood of Dicotyledons is, by sliding 

 growth, or unequal development of the individual cells. The wood is 

 consequently of that even texture seen in " deal." The tracheides are 

 lignified, and their radial walls are marked by circular bordered pits. 



The bordered pit which is found widespread in the tracheae of vascular 

 plants, is seen in perfection in the wood of Conifers. The pit originates as a 

 circular area of wall which remains thin, while the rest of the wall thickens. 

 But the thickening encroaches upon the area of the pit, and overarches it 

 (Fig. 248). As seen in surface view a double outline then appears. The 

 outer circle corresponds to the area of the pit-membrane, the inner to the limit 

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

 will be apart. Meanwhile the centre of the pit-membrane itself thickens, 

 forming the " torus," which serves mechanically to meet the risk of 



FIG. 248. 



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

 with pits in its radial walls. B shows the torus pressed to one side. C, D, E illustrate 

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

 as seen in surface view. E shows this in the young state. D rather more advanced. 

 C, mature. 



rupture following on any unequal pressure on the two sides. For the torus 

 would, as the membrane yields, press against the overarching lip (B). The 

 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 fluid between thick-walled cells and the maintenance of 

 mechanical strength For the former a large pit-membrane is an advantage, 

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

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

 maintained. 



The phloem is also arranged in regular radial rows. It consists chiefly of 

 sieve-tubes (v, Fig. 249) 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 by 

 medullary rays, as in Dicotyledons (Fig. 249). 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. 



