FIBROVASCULAR TISSUES: PARENCHYMA 



57 



In other coniferous woods, particularly those higher in the scale 

 than Pinus, the parenchymatous elements, whether radial or 

 longitudinal in position, secrete antiseptic substances such as 

 essential oils, tannin, etc., which preserve the heart structures 

 from the decay resulting from the attacks of wood-destroying 

 fungi. Fig. 43 throws light on the similar conditions as regards 

 the organization of heart and sap in the oak, as an example of the 

 dicotyledons. To the right, radial and longitudinal parenchymatous 



FIG. 43. Heart- and sapwood in the oak. Explanation in the text 



elements appear loaded with starch and provided with proto- 

 plasm and nucleus. The radial storage cells are represented with 

 light walls, while the true wood parenchyma is delineated with 

 thick, black, bounding membranes. Larger and smaller vessels 

 are to be seen corresponding to the spring and summer region of 

 the wood. The rest of the area is occupied by tracheids (larger and 

 thinner-walled) and fiber-tracheids (narrower and with thicker 

 walls). Turning now to the left, where the organization of the 

 heartwood is indicated, we discover the same absence of contents 

 in the radially and longitudinally directed elements with simple pits 

 (in other words, the parenchymatous elements) as in the case of 

 the similar structures of the pine. In this instance the larger 

 vessel presents a certain analogy to the resin canal of the conifer by 

 reason of the fact that it is occluded by an ingrowth in the form of 



