664 



A TEXTBOOK OF THEORETICAL BOTANY 



where it lies in contact with the dome, the torus fitting over and stopping up 

 the opening on that side and thus saving the membrane from rupture. 



The woody tissue of Pinus contains predominantly only one type of 

 conducting cell, the tracheid, though the primary and secondary tracheids 

 differ in the pattern of their thickening, but in the earliest primary xylem 

 there are very long spiral elements which appear to be true vessels. If this 





Trace of short shoot 





^ 



' Cortex 



Phloem 



Fig. 663. — Pinus sylvestris. Transverse section of a young 

 branch showing an early stage in secondary thickening. 



is the case their absence from the mature wood is a matter of some evolutionary 

 interest. 



As each tracheid is a closed cell, the water passing through the stem must 

 traverse thousands of bordered pits on its way, and their aggregate resistance 

 to the flow must be considerable. The reduced size and sclerotic character 

 of the leaves have often been attributed to this resistance in the xylem, 

 limiting the water supply to the foliage. Measurements made by Farmer 

 of the relative conductivity of woods in many species bear this out. The 

 conductivity of Conifer wood averages about one-half that of evergreen 

 Angiosperms and less than one-quarter that of deciduous Angiosperms, that 

 is to say, those which drop their leaves in winter. 



The protophloem is soon crushed out of existence by the pressure of 

 the growing tissues. The metaphloem consists of short sieve tubes with 

 sieve plates on their radial longitudinal walls and a few on their oblique 

 end-walls. Phloem parenchyma cells are also formed, but no companion 

 cells. 



Between the primary bundles are the parenchymatous primary medullary 



