282 



TISSUE ELEMENTS OF SEED PLANTS 



Both these types are formed in tissue which is still 

 elongating, and the stretching of the thin cell wall 

 separates the coils of the spiral or the annuli, as the 

 case may be (Fig. 45, B G). 



(c) Pitted tracheids have general thickening of the 

 cell wall, but with numerous pits often set so close 

 together (Fig. 45, J) that only ribs of thickening are 

 formed between them. The pits are generally bordered, 

 often with greatly overhanging walls (Fig. 45, I). When 

 the pits are elongated and horizontal in direction, as 

 seen in surface view, running across one face of a poly- 



FIG. 46. Part of a spiral tracheid very highly magnified and seen 

 in longitudinal section, showing attachment of the spiral thickening 

 band to the thin wall. 



gonal tracheid, so that there are horizontal bars of 

 thickening between successive pits, like the rungs of a 

 ladder, the tracheid is called scalariform (Fig. 45, H). 

 When the pits are angular and variable in outline, so 

 that there is a network of thickening between them, 

 we have the veiiculate type (K). The pits are, however, 

 very often circular, oval or polygonal in outline and 

 very closely set (J). 



A vessel has its walls lignified and thickened just 

 like a tracheid, but it consists of a row or chain of 





