STEMS 



697 



hundred millimeters or more. Bast fibers are multinucleate from 

 the outset {i.e. they are coenocytes) and they remain alive much longer 

 than do tracheids or tracheae. The walls are highly thickened with 

 cellulose or occasionally with lignin, the material being deposited in 

 regular centripetal layers; the stratification incident to differential 

 or periodic deposition often is very evident (fig. 1020, A). In the mature 

 fiber the lumen is extremely small, the volume of the wall being many 

 times greater. The walls are marked by spirally arranged slits, sug- 

 gesting that the ultra-microscopic particles {micellae) also are spirally 

 arranged like the strands of a rope. Wood fibers (also called libriform 

 elements) resemble bast fibers, differing therefrom in their restriction to 

 secondary wood, and in their lignified walls and early death; further- 

 more, they are not coenocytes. There are all gradations between wood 

 fibers and tracheae, whereas bast fibers are contrasted rather sharply 

 with other phloem elements. 



Collenchyma. — Collenchyma is a term applied to living mechanical 

 tissues made up of elongated cells whose walls are unequally thickened 



Fig. ioji. — A cross section of a 

 Begonia petiole, showing collenchy- 

 matic thickening (c) of the walls at 

 the angles of the outermost cortical 

 cells; t, cuticle; highly magnified. 



Fig. 1022. — A stone cell or sclereid 

 from the petiole of the wax plant {Hoya 

 carnosa) ; note the lines of stratification 

 (5), representing successive periods of 

 wall construction, and also the canals 

 (c) which connect the lumina of adjoin- 

 ing cells, and which have remained open 

 during wall-building; highly magnified. 



with cellulose of high water content (sixty to seventy per cent, as against 

 twenty to forty per cent in bast), the thickening often being most pro- 

 nounced at the cell angles (fig. 102 1). Unlike bast, collenchyma is 



