27] CHAPTER II. THE TISSUES. 89 



so that the individual cells may be separated. When the common 

 wall of similar cells is pitted, the pits on each side accurately 

 meet (Fig. 66 ) ; if, however, certain cells of a tissue undergo a 

 special modification, as in the vessels, the unequal thickening of the 

 membrane may be confined to one side only of the common wall ; 

 in the case of spiral thickening of the cell-wall this is self-evident. 



In certain cases the septa between the cavities of adjacent cells 

 become wholly or partly absorbed, as, for instance, occasionally the 

 thin partition between bordered-pits ; the transverse walls of such 

 cells as combine to form the vessels are wholly absorbed, if they lie 

 at right angles to the long axis of the vessel (Fig. 73 C a V) ; if 

 they lie obliquely, they are broken through in various ways. In a 

 similar manner the transverse septa (and more rarely isolated areas 

 on the longitudinal wall also) of the sieve-tubes ( 28, Fig. 74 B) 

 are perforated by closely-set and very fine open pits, and are then 

 known as sieve-plates. Hence a vessel is a syncyte (see p. 64). 



27. Intercellular Spaces are lacunae between the cells of 

 a tissue. They may be formed in two ways : either by a splitting 

 of the common wall of adjacent cells, that is schizogenously ; or by 

 the disorganization of certain cells, that is lysigenously. They 

 contain either air or certain peculiar substances. 



The intercellular spaces which contain air are usually formed 

 schizogenously. They occur almost exclusively in parenchymatous 

 tissue, at the angles of junction of a number of cells (Fig. 67 z)> 

 Sometimes these 

 spaces then called 

 air-chambers at- 

 tain a considerable 

 size, so that whole 

 masses of tissue are 

 separated from each 

 other, as in the 

 petioles of the Water 

 Lily and of other 

 aquatic plants. 



The large cavities 



in the Stems and FlG- 6 7.-Intercellular spaces (*) between cells from the 



leaves of Rushes and stem of Zea Mais ( x ;60) ; gto the common wall. (After 



of other allied plants, 

 are produced lysigenously by the drying-up and [rupture of con- 

 siderable masses of cells : this is true also with reference to the 



