MOKPHOLOGY OF THE CELL. 



The cambium-cells and the youngest tracheids have uniform 

 and smooth walls, but in those next older there appear thin 

 spots, which are well defined above 

 and below, but not on the sides, for 

 here the}- grade off into the thicker 

 part of the wall. In the cells which 

 are still older the thin places take the 

 shape of discoid markings, and are 

 clearly seen in an)' radial view. Com- 

 parison of radial with transverse sec- 

 tions shows that at the margins of the 

 thin places a portion of the wall ex- 

 tends as a slight projection upwards, 

 and partly over the spot. In the more 

 mature form the thin place is still re- 

 tained as a delicate plate separating 

 the two cells, but easily broken down 

 perhaps in further growth. 



270. Scalariforni markings (see 

 134) are especially abundant in ferns. 

 The bordered pits are much elongated, 

 and appear as clefts with only narrow 

 portions of the wall between them 

 (Fig. 64 I). They often follow each 

 other with as much regularity as the 

 "rounds" of a ladder, whence the name (from scalaria, a 

 flight of steps). They are more commonly found in 



DUCTS. 



271. Ducts, or Tracheae, are variously marked by pits, and 

 by the thickenings described in Chapter I. Some of the more 

 common forms of dots are shown in Fig. 64. 



Spiral, annular, and reticulated markings are all formed by 

 the thickening of parts of the wall by which narrow lines or 

 bands are produced on the inner surface. In these cases the 

 portions of the wall which are not thickened are often of extreme 

 tenuity, and break upon slight pressure or strain, permitting the 

 spiral to uncoil or the rings to separate (Fig. 64, s s'). 



272. Spiral markings. The number of threads or narrow 

 bands varies from one to fifteen or even twenty, the latter in the 

 petioles of Musa. 1 They wind, as a rule, from right to left ; 



1 De Bary : Vergleichende Anatomic, 1877, p. 163. 



FIG. 63. Pinus sylvestris. Cross-section through the cambium and young wood-celU. 

 (Strasburger.) 



