THE ANATOMY OF WOODY PLANTS 



will be observed that there are well-developed pits toward the inter- 

 cellular aerating spaces. These are known as air pits and have 

 no counterpart on the side of the air space. They are commonly 

 found wherever the thickening of the cell wall makes difficult the 

 interchanges between the living protoplasm and the outside air. 

 In the figure under consideration the cells are united by the cement 

 substance of the middle lamella, which appears dark and contains 



the air spaces within its sub- 

 stance. Most of the cells 

 have had both the top and 

 the bottom walls removed 

 by the plane of section, but 

 in one or two the cell wall 

 meets the eye. Here the 

 pits are seen in face view, 

 and it becomes clear that 

 they vary somewhat in size 

 and are characteristically 

 outlined by a single contour. 

 On account of this condi- 

 tion, which results from the 



FIG. 3. Section of inner pericarp 

 ("stone") of a peach, showing cells with ex- 

 tremely thick and layered walls as well as 

 numerous elongated pits. 



fact that the bounding walls 

 do not overhang, pits of this 

 type are called simple pits. 

 Communicating pores of this kind are found in cells which in 

 their functional condition contain living protoplasm. 



Let us now turn to the examination of cells in which the wall 

 is so greatly thickened that the space containing the protoplasmic 

 body becomes much reduced in size. Fig. 3 reproduces some of 

 the elements of a peach stone which has been softened by a pro- 

 longed sojourn in hydrofluoric acid. The protoplasmic structures 

 have been largely consumed in the thickening of the cell wall. 

 The latter shows well-marked indications of layering, a feature 

 often present when much thickening has taken place. The layer- 

 ing doubtless has some numerical relation to the age of the wall in 

 days. The pits are of special interest in this instance. Their 

 correspondence in adjacent cells is as marked as in the former 



