MORPHOLOGY 



87 



cytoplasm. These cytoplasmic threads penetrate the cell walls, and 

 in particular the partition membranes of their pits (Fig. 99). It may 

 be inferred that the conduction of stimuli 

 from one cell to another is carried on by 

 means of these cytoplasmic connections. 

 Viewed thus, the whole plant becomes a 

 living unit. Between cells having such 

 cytoplasmic connections and a fusion of 

 cells, such as a sieve-tube, there is little 

 distinction. In this sense a whole plant 

 forms a single cell fusion, although incom- 

 plete and limited by cell walls. 



The cells in a tissue may either fit 

 closely together, leaving no openings or 

 spaces, or so-called INTERCELLULAR SPACES 

 (inteecellulars) may be left between 

 the individual cells. 



Where cell filaments are interwoven 

 into a tissue, their intercellular spaces are 

 represented by the openings left between 

 the loosely-intertwined filaments (Figs. 95, 96). In tissues resulting 

 from cell division the intercellular spaces arise subsequently, as the 



Fig. 95. — Longitudinal section of 

 the stalk of the fructification of 

 Boletus ednlis. (X 300.) 



Fig. 97. — Longitudinal sec- 

 tion of the selerotium of 

 a. (XS00.) 



Fig. 9S. — Transverse section of 

 the selerotium of ClavUxps 

 purpurea. (X 300.) 



Fig. 9(5. — Transverse sec- 

 tion of the stalk of the 

 fructification of Boletus 

 edulis. (X 300.) 



partition wall between two cells formed by cell division originally 

 belonged to both mutually. 



Such a partition wall may ultimately split and so give rise to intercellular 

 spaces, but this only occurs after it has been thickened. The cause of such 

 splitting is to be found in the hydrostatic pressure existing within the cells, and 

 their consequent tendency to assume a spherical shape. The formation of inter- 

 cellular spaces commences, therefore, at the cell corners, where the primary wall, 

 consisting of pectinose material, becomes swollen. 



The simplest and at the same time most frequent intercellular 



