SPIRAL AND ANNULAR TRACHEIDS 



28l 



cell through its entire length. The successive turns of 

 the spiral are at first in close lateral contact (Fig. 45, A). 

 The wall itself, apart from the thickening, usually 

 remains thin and cellulose. 



H 



FIG. 45. Water-conducting tissue elements. A, spiral tracheid, not 

 yet elongated, surface view. B, elongation beginning, separating 

 coils of spiral thickening. C, portion of B more highly magni- 

 fied, showing nucleus and cytoplasm still alive. D, part of 

 tracheid with coils of spiral further separated. E, ditto of 

 annular tracheid. F, spiral tracheid with cavity nearly 

 obliterated owing to pulling out of spiral thickening till it no 

 longer supports the thin wall. G, ditto in annular tracheid. 

 H, part of scalariform vessel. I. bordered pits : in (a) sectional 

 view, (b) surface view (corresponding points joined by horizontal 

 lines). J, part of pitted vessel. K, part of reticulate vessel. 



(b) Annular tracheids (Fig. 45, E, G) are similar, but 

 have the thickening in the form of separate rings of 

 lignified substance, set one above the other, at first 

 in close contact. 



