THE GYMNOSPERMAE : CONIFERALES AND TAXALES 663 



meristematic cells, the apex of which is occupied by an actively dividing group 

 of initial cells (Fig. 661). From these are derived two zones of tissue, a 

 central zone of large, pith-forming cells and an outer zone which corresponds 

 morphologically to the tunica (see p. 850), although its cells do not only 

 divide anticlinally. In this outer zone there arise a number of procambial 

 strands, in which certain radial rows of cells become differentiated into 

 protoxylem and protophloem at their inner and outer ends respectively. 

 At a later stage the median band of undifferentiated procambial cells in each 

 strand becomes the cambium, from which metaxylem is formed internally 

 and metaphloem externally. All these together constitute a vascular 

 bundle of primary xylem, cambium and primary phloem. 



A 



B 



C 



Fig. 662. — Pinus sylvestris. Bordered pits. A, B and C, 

 Stages in the growth of border and torus in three- 

 dimensional view. D, Bordered pits in transverse 

 section. E, Bordered pit with torus displaced laterally, 

 filling aperture of pit. F, Pit membrane in surface view 

 with central torus and radial striations. {After Stevens.) 



The walls of the protoxylem elements show a loosely spiral thickening 

 with a few small bordered pits. The metaxylem elements are thickened 

 with either a close spiral or a close reticulum, while the bordered pits become 

 larger and more numerous in the later formed elements. 



A bordered pit is a circular area of unthickened middle lamella, which 

 is over-arched on each side by a dome formed of the inner, lignified layer of 

 the cell wall (Fig. 662). Each dome has a small central opening. On the 

 lamella, and in line between the two openings, there is a small disc of lignin 

 called the torus, around which radiate a number of openings in the lamella, 

 like the spokes of a wheel. The torus acts as a stopper, for any marked drop 

 of pressure on one side of the pit will draw the lamella over to that side, 



