ECONOMIC WOODS OF THE UNITED STATES 131 



cavities of adjacent cells and in some instances with intercellular 

 spaces also. 



Where the cell walls are thin enough there is no need for special 

 provision for intercommunication. The process of thickening 

 reduces the permeability of the walls and makes necessary the 

 leaving of thin or unthickened spots called pits. Were the wall 

 uniformly thickened throughout, the lumen would become isolated 

 and the function of the cell would be reduced to that of reinforce- 

 ment only, a condition approximated in the libriform fibres of 

 certain woods such as Toxylon and Robinia. At the other ex- 

 treme, there are elements (vessels) concerned with the rapid con- 

 duction of water which are composed of vertical series of cells 

 whose pits at the ends have given place to true openings or per- 

 forations. The only fundamental difference between a perforation 

 and a pit is that in a pit the middle lamella, somewhat modified, 

 forms a limiting or pit membrane. The presence of minute per- 

 forations in this membrane can be demonstrated by passing finely 

 divided solid particles through it. 



Some cells have simple pits while others appear under the 

 compound microscope to have a more or less distinct border. 

 This border is due to the wall overhanging the margin of the pit 

 membrane. Pits between food-cells are simple while those be- 

 tween water-carriers are bordered. Where the two types of cells 

 are in communication the half of the pit in the food-cell is always 

 simple and the corresponding portion in the other may be either 

 simple or bordered. In the latter case the pit is structurally half- 

 bordered, though in surface view it may not be distinguishable 

 from one that is bordered on both sides. Pits exhibit a wide range 

 of variation in size, shape and arrangement, and possess high value 

 for purposes of classification of woods. (For further details see 

 pp. 31-35.) 



CLASSIFICATION OF THE ELEMENTS OF SECONDARY WOOD 



On p. 13 the cellular elements of wood are referred to three 

 principal types, viz., vascular, fibrous and parenchymatous. 

 Some authors prefer the following classification: (a) vessels (cell- 

 fusions serving for the conduction of water) ; (b) parenchyma (food 

 cells which conduct and store carbohydrates); (c) prosenchyma 

 (cells serving chiefly to give mechanical support but often partici- 

 pating in functions of the other groups). The prosenchyma in- 



