SECT. [ 



MORPHOLOGY 



73 



the thickening hiyers exhibit delicate striations in surface view. The striations 

 extend through the whole thickness of the layers, usually running obliquely to 

 the long axis of the cell, and often crossing one another in the different thickening 



nv 



Fig. 76. — From the wood of the Pine, Finus sylrestris. A, 

 Bordered pit in surface view ; B, bordered pit in tan- 

 gential section ; t, torus ; C, transverse section of a 

 tracheide ; m, middle lamella, with gusset, m* ; i, inner 

 peripheral layer, (x 540.) 



Fig. 77.— Cells from the endosperm 

 of Ornitliogalum umbdlatum. in, 

 Pits in surface view ; p, closing 

 membrane ; n, nucleus, (x 240.) 



layers (Fig. 73). The striations depend on the presence of lamellae of different 

 thickness, the thicker ones projecting slightly into the cell cavity. A similar 

 appearance of crossing of striations may result from the single thickening layers of 

 two adjoining cells being visible at once. 



Fig. 78. — Sieve-plates on the ter- 

 minal wall of a segment of a 

 sieve-tube of the Scotch Fir. 

 (X 750.) 



Fig. 79. — ^1, Part of an annular tracheide ; B, part 

 of a spiral tracheide ; C, longitudinal section 

 through part of a reticulate vessel .showing the 

 remains of a partition wall, s. (x 240.) 



The thickening of the cell wall seldom takes place uniformly 

 over the whole surface ; but some portions are thickened, while, at 



