THE TRACHEIDS 



109 



windward 



Fic. 53. — Diagram to show 

 the tangential flow of water 

 from the side where there is less 

 demand to the side where the 

 demand is greater. 



of Strengthening the wall where it is weakened by the thin place, 

 and of arresting and bracing the thin part when from unequal 

 pressure at one side or the other it is in danger of bulging too 

 far and bursting. 



In the early growth of the annual 

 ring of pine the bordered pits are 

 almost exclusively in the radial walls, 

 while in the late growth they occur 

 for the most part in the tangential 

 walls (Fig. 52). This difference 

 seems to relate to certain physio- 

 logical demands. If for any reason 

 one side of the tree — the windward 

 side, for instance — demands more 

 water than the others, the water ris- 

 ing through the trunk can pass to 

 that side through the pits in the 

 radial walls, as shown in the diagram (Fig. 53). And since the 

 medullary rays extend individually but a fraction of a milli- 

 meter vertically the water can find its way around the stem 

 without traversing them, as illustrated by Fig. 54. Again, when 



, I the new crop of leaves is produced 



in the spring and the cambium 

 commences the new ring of growth, 

 beginning in the crown and pro- 

 gressing toward the roots, the 

 bordered pits in the tangential walls 

 of the late growths of the previous 

 summer permit water to pass into 

 the tracheids of the new growth as 

 the latter progresses toward the 

 roots and is not yet in position to 

 draw directly upon the roots themselves (Fig. 55). 



How the boardered pits assist in the flow of the water ver- 

 tically, which without doubt is their chief function, is made 

 clear by a study of longitudinal tangential sections where it 



Fig. S4- — Diagram showing by 

 the arrows how the water can flow 

 tangentially around a plant without 

 traversing the medullary rays, 

 which are indicated in black. 



