244 STRUCTURAL BOTANY 



cells, so that no communication is possible between 

 them except through their pits. The whole of the 

 water which goes up a Fir-tree has to pass through 

 the bordered pits thousands of times on its journey 

 from the roots to the leaves. The thin membrane is 

 protected from rupture by the torus. When the 

 pressure is much higher in one tracheide than in its 

 neighbour, the closing membrane is bulged out, be- 

 coming convex on the side towards the lower pressure. 

 Thus the torus is pressed closely against the opening 

 of the pit border on that side (Fig. 99, B). The thin 

 part of the membrane is thus supported by the border, 

 and only the thickened torus is exposed to the full 

 pressure. 



The border has another important function ; it 

 serves to keep the closing membrane moist. In 

 times when transpiration is very active, and the water- 

 supply deficient, as in hot, dry summers, the tracheides 

 become very nearly emptied of water. Within the 

 minute cavities of the bordered pits, however, the 

 water is held fast by capillary action, and so the 

 closing membranes are prevented from drying up. Thus 

 the entrance of air into the tracheides is guarded against. 

 This is a very important matter, for if the tracheides 

 once became filled with air, no more water would be 

 able to pass through them. 



The wood is not uniform all through. As everybody 

 has seen, the wood of the Fir, like that of other trees 

 in temperate climates, shows annual rings, each ring 

 representing a year's growth, so that by counting 

 them the age of the tree can be ascertained. 



