22 THE PHYSIOLOGY OF PLANTS 



the cells and vessels, except in the case of extreme drought. 

 The water which is contained in the interstices of the mem- 

 brane of one cell or vessel can be partly attracted by the wall 

 of an adjoining vessel, and so the moisture will spread evenly 

 through the walls of these two adjoining elements. When 

 the cell contents can part with no more moisture, the cell- 

 walls can still effect an interchange among themselves. This 

 is true not only of the vessels, but also of all lignified cells. 



We cannot conclude the consideration of this part of the 

 subject without referring to the occurrence of special valves 

 which we find in this arrangement of suction and forcing 

 pumps. The valves are represented m this water-conducting 

 tissue by thin portions of the cell-walls, which are usually 

 termed bordered pits. 



In the cells of all the tissues which make up the vegetable 

 body, we can easily recognise, even if the walls are only 

 slightly thickened, that they are made up of two layers, which 

 are physically and chemically different one from the other. 

 We can recognise an outer thin but hard membrane, which 

 was the first to be formed, and is hence termed the primary 

 membrane. Within the latter will be seen a less resistant 

 but thicker layer, which seems to form a padding on the 

 inside, and which is called the seeondary membrane. This 

 layer, however, is not of the same thickness throughout, but 

 presents smaller and larger areas, at which it seems more or 

 less interrupted. At such points the thickening of the cell- 

 wall, which always takes place when a cell passes from its 

 youthful condition into its mature state, may have been 

 defective, or may not have taken place at all. If these areas 

 have the appearance of small holes or pits in the secondary 

 membrane, the cells or vessels are termed "pitted;" if the 

 apertures are numerous and large, so that the secondary 

 membrane has the appearance of a net spread over the inside 

 of the primary membrane, we speak of reticulate elements. 

 In scalariform, annular, or spiral cells or vessels, the secondary 

 membrane presents a ladder-like, annular, or spiral appearance. 



We shall get an idea of the method of thickening of 

 the cell-wall from the representation of a thick vessel cut 

 longitudinally in Fig. 7 {lower g). The figure represents a 



