INTRODUCTION TO PLANT HISTOLOGY 463 



description of woods. The tracheids which most resemble vessel elements 

 (see below) are called vascular tracheids. They have the same kind of lignifica- 

 tion as vessels and where they abut on vessels the pits between them are the 

 same as those on the vessel. Tracheids which more resemble fibres are 

 classed as fibre tracheids, though they may have the same kind of ring or 

 spiral thickenings as vascular tracheids. The presence or absence of a border 

 to the pits is often taken as a line of demarcation between tracheids as a class 

 and true fibres, tracheids being defined as cells in which the pits to elements 

 of the same type are bordered, while the fibres have only simple pits. This 

 is not a precise boundarv line, for the amount of bordering to a pit is very 

 variable. 



The best that can be said is that tracheids have definitely bordered pits, 

 while the true fibre has usually simple pits, and is an elongated and commonly 



Fig. 450. — Growth of a tracheid from a cambial cell, showing 

 progressive stretching due to vertical tension in the tissues. 

 {After Lezvis.) 



thick- walled cell, which in the sap wood often contains starch and is not 

 infrequently transversely septate by thin cross walls, which are formed after 

 the side walls have been thickened. 



The first tracheids to appear are differentiated very near the growing point, 

 in the midst of cells which are actively expanding. A dead cell, such as a 

 tracheid, is subject therefore to considerable lateral pressure from its neigh- 

 bours and also to longitudinal strain from the elongation of the cells which 

 are in contact with it. In such cells the lignified thickening takes either the 

 form of transverse rings, or of one or more spirals round the cell, between 

 which is the thin, cellulose w'all (Fig. 451). This form of thickening is 

 associated with the maximum of extensibility combined with considerable 

 resistance to compression. These types are known as annular and spiral 

 respectively, and thev are characteristic of the first formed wood elements. 

 Tracheids formed somewhat later show instead an anastomosing network of 

 lignin bands and are called reticulate, while those developed in mature 



