24 THE PHYSIOLOGY OF PLANTS 



widest vessel lying farthest to the right (lower g), which has 

 been cut longitudinally, so that its interior is visible. First 

 of all we are reminded by the figure of the origin of vessels 

 from superposed cells ; for we can still see the several barrel- 

 shaped segments, the transverse ends of which run round the 

 inside of the vessel. Each such ring consists of two light- 

 coloured rings separated by a dark line. This latter repre- 

 sents the original cell-wall (primary membrane) which separated 

 the two cells, and upon it the lighter-coloured thickenings 

 (secondary membrane) have gradually become deposited. This 

 same layer of thickening has, however, not been so evenly 

 deposited over the remainder of the vessel as it was on the 

 rim of the original transverse walls, but is perforated by small 

 funnel-shaped pits, the structure of which will perhaps be 

 best understood by an examination of the lateral walls, which 

 are the only portion preserved in the upper portion of the 

 vessel. These walls appear to consist of a thin continuous outer 

 wall beset with a number of beads. This bead-like arrange- 

 ment is exhibited by the section of the secondary membrane 

 which is perforated by the pits, the space between two of 

 these bead-like bodies representing the pit itself, the broader 

 end of which lies towards the primary membrane. We see, 

 therefore, that these pits of the vessel are not actual pores, 

 but only holes in the inner secondary membrane, covered in 

 by the thin outer or primary membrane of the vessel. 



This process of thickening, which takes place with the 

 maturing of the cells, is an absolute necessity. For how 

 should such a colossal growth of millions of cells piled one upon 

 the other preserve its shape if the cells remained in their soft 

 and youthful condition ? The whole structure would collapse 

 by its own weight into a shapeless mass. 



Hence it becomes necessary to strengthen the separate cells 

 and vessels, and this is done by the formation of the secondary 

 membrane on the inside of the cell-wall. But it is also 

 useful, if not necessary, that this secondary membrane should 

 not be continuous, but should leave pores or pits at intervals, 

 so that the water may pass easily tlirough the thinner portions 

 of the cell-wall, for its passage through the thicker portions of 

 the cell-wall could only be accomplished with great difficulty. 



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