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Practical Plant Biology. 



flexibility of the membrane allows it to be displaced, so that it 

 may lie against the dome on either side, while the torus blocks 

 the aperture of the dome. As will be explained in a later lecture, 

 these peculiar little valves play an essential role in enabling the 

 evaporating leaves to draw up a continuous supply of water through 

 the plants. 



A comparison of the appearances presented by the various struc- 

 tures composing the wood in sections taken in different directions 

 will bring out clearly their relations and will enable you to recon- 

 struct mentally the solid form of each. For this purpose it is neces- 

 sary to examine the appearance of the tracheids, medullary rays, 

 and bordered pits in three sections taken at right-angles to each 

 other, viz. in transverse, radial and tangential sections. 



FIG. 58. Finns silvestris, plan (in middle), section (at either side) of bordered 

 pit, x about 1000. d, dome ; /, torus ; m, membrane ; r, radial wall of 

 tracheid ; t, tangential wall. The section on the left shows the mem- 

 brane and torus in a median position; that on the right shows them 

 drawn against the dome. 



(i) Transverse section : 



The whole of the secondary wood of Pinus, i.e. all the wood 

 except an insignificant portion immediately round the pith, is built 

 up of tracheids. Each cambial cell originates a radial row of them 

 so that in transverse section we see their rectangular cross sections 

 forming radial series. The regularity of these series is only dis- 

 turbed if the section is at the level of the overlapping ends, or if, 

 as occasionally happens, the cambial cell divides radially, and the 

 radial series becomes double. As you have already learned the 

 walls of the summer tracheids are thicker than those formed in 

 the spring. In transverse sections these thick-walled tracheids 

 form dark bands crossing the radial rows at right angles. These 



