246 STRUCTURAL BOTANY 



The elements of which the rays are composed are of 

 two kinds. Some are living parenchymatous cells 

 in which starch is formed (see Pig. 98 , s). Others, 

 however, completely lose their living contents, and 

 become tracheides (see Fig. 98, t, t). These have 

 bordered pits like the long tracheides of the wood, and 

 their walls are irregularly thickened. These trach- 

 eides in the ray serve for the passage of water in 

 the radial direction, so that by their help communi- 

 cation is kept up between the different layers of the 

 wood. The parenchymatous ray-cells have only 

 simple pits, except where they are in contact with 

 tracheides, in which case the pits are bordered on 

 the side towards the tracheide. The parenchymatous 

 cells serve for the storage of reserve food-substances, 

 especially starch and oil. The two kinds of elements 

 in the ray are arranged in horizontal rows. Often we 

 find rows of tracheides at the top and bottom of the 

 ray, and parenchymatous cells in the middle. 



The resin canals which traverse the wood resemble 

 those already described; their cavity is surrounded 

 by a layer of thin-walled secreting cells. In each 

 of the larger medullary rays a horizontal resin canal 

 is usually found. These horizontal canals are in 

 communication with the vertical ones. 



Summing up the peculiarities of this example of 

 coniferous wood, we find that its great characteristic 

 as compared with the wood of a Dicotyledon is its 

 less perfect differentiation. Vessels and woody fibres 

 are both absent. The tracheides have to do duty 

 for both, those of the spring wood taking the chief 



