78 THE PLANT SKELETON 



bundles. It may be in isolated longitudinal strands or in the 

 form of an unbroken hollow cylinder (Fig. 17). 



The walls of the bast fibers become much thickened, even in 

 extreme cases to the entire closing of the cell cavity. The length 

 of the fibers varies within wide limits: in some cases they are no 



j 



more than i mm. long, while in flax they reach a length of 40 

 mm. and in hemp 77 mm. The average length is about 2 mm. 

 It will readily be seen by these figures how it is that the fibers 

 of flax and hemp can be twisted into thread and woven into 

 cloth, while those of most plants are much too short for the 

 purpose. The length of the fibers has much to do with the 

 strength of the bast tissue as a whole, for, as has already been 

 said, as the nascent fibers elongate their ends glide by one an- 

 other, and the length of the splice increases with the length of 

 the fibers. As a rule the walls of the bast fibers become decidedly 

 lignified, but all grades of condition occur from almost pure 

 cellulose to complete lignification. 



In those herbaceous plants whose cambium produces little or 

 no wood fiber tissue the bast remains the chief dependence for 

 strength; but where the wood is much represented as in the 

 older parts of many annual stems and in all woody perennials, 

 the significance of the bast as a strengthening tissue falls into 

 the backgrouud. This fact is quickly appreciated by noticing 

 how little in such cases the strength of the stem is diminished 

 by stripping off the bark. 



The bast fibers are employed not only to strengthen the stem 

 as a whole, but also to protect and give stability to the delicate 

 tissues of the primary and secondary phloem, as when the bast 

 strands stand like a buttress before the primary phloem or, in 

 the secondary phloem, are built by the cambium alternately 

 with groups of sieve tubes, companion, and parenchyma cells. 

 In Monocotyledons a fibrous tissue similar to the bast of Dicoty- 

 ledons surrounds each isolated vascular bundle more or less 

 completely (Fig. 40). 



The bast fibers are especially fitted for their mechanical 

 function by their great elastic strength. (By elastic strength is 



