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Practical Plant Biology. 



is the cortex which is composed of cylindrical cells with inter- 

 cellular spaces between them, their axes being parallel to the 

 axis of the stem. They are thin-walled and the outer cells 

 contain chlorophyll. Starch grains are usually found in all. 

 Between the conducting tracts and separating them from one 

 another are (2) the medullary rays. They are composed of 

 cubical or oblong cells with thin walls, containing stores of 

 carbohydrates. They connect the fundamental tissue of the 

 cortex with that of (3) the pith, which forms the axial core of 

 the stem. The pith is built up of rounded cells, thin-walled 

 and usually containing store materials, and enclosing between 

 the cells intercellular spaces. The intercellular spaces through- 



FIG. 54. Pinus silvestris, young long shoots, transverse sections, x 7. 

 Section on the left taken at the beginning, that on the right towards the 

 end of the first season's growth, a, cortex ; b, resin-ducts ; c, bast or 

 phloem ; d, cambium ; e, medullary ray ; /, wood or xylem ; g, pith. 



out the fundamental tissue form a continuous system and so 

 allow of a slow interchange of gases between the inner tissues 

 and the outside. Throughout the stem and leaves there is 

 another system of passages in the form of branching-tubes lined 

 with cubical cells which exude resin into the tubes. These 

 resin -passages are found in the wood, in the medullary rays 

 and in the cortex of the stem and of the mesophyll of the 

 leaf. Being surrounded by turgid cells the resin in these ducts 

 or passages is under pressure, and when the tree is wounded 

 is extruded through the surface of the wound. In these cases 

 the resin appears to act as an antiseptic and coating the wound 

 prevents excessive loss of water by evaporation. 



