38 



BOTANY. 



32. — The passage from the mode of thickening just de- 

 scribed to the scalariform manner (Fig. 18) is an easy one. 

 Here each longitudinal angle of the cell or vessel is thickened, 

 and from these thickened angles ridges run right and left, 

 from one to the other (Fig. 18, C, v). The after growth 

 of the ridges is essentially the same as in the case of crowded 

 pits ; in fact, the pits here are simply greatly elongated and 

 crowded bordered pits. Eventually the narrow plates be- 

 tween the thickened ridges disappear, as in the other cases. 

 Examples of scalariform thickening are common, especially 

 in, the ferns. 



33. — Thp development of rings (Fig. 19, v) is nearly like 

 that of the scalariform thickening. Instead, however, of 



I'ig. 19.— Loneitndinal section of a portion of ^ihaismot ImpatleneBaltamina. 

 «, annular vessel. v\ a vessel with thickenings which are partly spiral and partly an- 

 nular ; ?/', i)"\ V"", several varieties of spiral vessels ; v""\ a reticulated vessel. — 

 After Duchartre. 



the ridges being short, they extend entirely around the inner 

 surface of the wall. The transition from rings to spirals is 

 a simple one, the thickening taking place in a spiral line, 

 instead of in one passing directly around the wall (Fig. 19, 

 v'' , v'"). Transitional forms are frequently found (Fig. 19, 

 v'), and many modifications and irregularities occur — e.g., 

 in the figure at v'"" is the form known as the reticulated. 



34.^In all the foregoing cases the marking of the wall 

 has been general ; there are some cases, however, where it 

 is localized. A good example of this is in the formation of 

 the pits of sieve-cells (Fig. 20). The horizontal walls, and 

 also areas upon the longitudinal ones, become thickened 

 reticulately, leaving rather large thin areas, as shown in 

 Fig. 20, qi q. After a while the thin areas become absorbed. 



