154 PLANT ANATOMY. 
which are clearly disclosed only in thin and carefully prepared 
sections (Semen,Colchici). 
In those cases also where thin places of the cell-membrane 
remain preserved only in extremely slight amount and extent, 
the growth in thickness does not take place through the simple 
deposition of new encircling scales or layers of cellulose. The 
ro) 
7 
™~ v 
Fie, 71. 
Fia. 71.—Spirally arranged pits, 
Fie, 72.—Areolated dots of the tracheids of fir-wood. 4, transverse section through 
the tracheids or wood-cells, the pits shaded ; Band C, schematic longitudinal sections; 
the spherical lines denoting the circumference of the pit and the border; D, two adja- 
cent pits cut in the direction of length, with the partition-wall still retained (Sachs). 
stratification, which is often highly remarkable, depends upon 
variations in the amount of contained water and in the condi- 
tions of tension of the individual layers ; those containing less — 
water and which are denser, stand out distinctly in consequence 
