THE INNER TISSUEkS OF PLANTS. 267 



surfaces, may consist either of successive rings, or continuouji 

 spiral threads, or networks, or structures between spirals and 

 networks, or networks with openings so far diminished that the 

 cells containing them are distinguished as fenestrated. Their 

 differences omitted, however, these structures have the common 

 character that, while supporting the coats of the vessels and 

 serving to restore their diameters after they have been com- 

 pressed, they also give special facilities for the passage of 

 liquids, both through the sides cf the transformed cells and 

 through their united ends, where these are not destroyed. 

 For one of these internal frame- works is not, as usually stated, 

 produced by the deposition of substance on the cell-mem- 

 brane, in the shape which the frame- work eventually assumes. 

 Were it so, this frame- work would have a thickness additional 

 to that of the cell- wall as previously existing, which it has not. 

 On comparing one of these cells longitudinally cut through, 

 with an adjacent cell of the kind to which it was originally 

 similar, we see that over every opening in the frame-work, the 

 wall of the cell is far thinner than the walls of the adjacent 

 cells: the cell-membrane at each of these openings being quite 

 bare, instead of being, as in adjacent cells, covered by a layer of 

 deposit. Hence this transformation of cells into sap-channels, 

 is in part the arrangement or re-arrangeraent of their sub- 

 stance in such ways as greatly to diminish the resistance to 

 the passage of liquid, both longitudinally and laterally. 



To attempt any physical interpretation of this change 

 is scarcely safe : the conditions are so complex. There are 

 many reasons for suspecting, however, that it arises from a 

 vauuolation of the substance deposited on the cell wall. It 

 rapidly deposited, as it is likely to be along lines where sap 

 is freely supplied, this may, in passing from the state of a 

 soluble colloid to that of an insoluble colloid, so contract as to 

 leave uncovered spaces on the cell-membrane ; and this 

 change, originally consequent on a physico-chenycal action, 

 may be so maintained and utilized by natural selection, as to 

 result in structures of a definite kind, regularly formed in 



