70 INTERCELLULAR SPACES [CH. 



there are gaps in the corners where three or more cell- 

 partitions converge. In other words, the cells have 

 partially separated here, as if partly torn asunder. 

 The separation may be found to have been carried to 

 a much further extent in leaves, where the cells are only 

 left contiguous along relatively narrow surfaces. 



These lacunae between the cells are called inter- 

 cellular spaces, and are formed by the partial separation 

 of cells, the common walls of which were previously in com- 

 plete continuity. This separation occurs by the splitting 

 of the common partition-walls, along the boundary plane 

 which theoretically separates what belongs to one cell 

 from what belongs to the other ; and in the thicker cell- 

 walls of sections we can see a distinct bright line running 

 midway between the cells, in which the transverse section 

 of the plane referred to must lie. This line is called the 

 middle lamella, and it is in fact the dissolution of this 

 middle lamella which determines the separation of the 

 cells. 



The degree of separation varies in different cases from 

 almost nothing to complete isolation of the cells. In 

 cork and usually in wood, for instance, we find no inter- 

 cellular spaces at all developed, or only very minute 

 ones here and there : they are small and usually tri- 

 angular in section in the soft tissues of seeds, fruits, stems, 

 roots, &c.; while in leaves, the pith of some rushes, the 

 softer tissues of many marsh and aquatic plants (e.g. 

 Myriophyllum, (Enanthe, Butomus, &c.) whole series of 

 cells become nearly completely separated, the intercellular 

 spaces being often larger than the cell-cavities, and in 

 some cases very much larger (e.g. Aroids, Musacea3, Water 

 Lilies, &c., where they are visible to the unaided eye). 



In all these cases the advantage of the spaces is to 

 allow gases, especially air, to obtain access to the cells, 



