THE BAST FIBERS 77 



Since the collenchyma is formed where growth in length of 

 the stem is still taking place it must be capable of growing 

 itself or of stretching and offering a moderate resistance only 

 to the increase in size of the other tissues. For this reason it 

 cannot be relied on as the mainstay very far down the stem 

 where the stresses to be overcome are greater than in the region 

 of the apex, and the bast fiber tissues are formed there to re- 

 enforce it (Fig. u). The elastic strength of the collenchyma 

 is small compared with that of wood and bast fibers, and when 

 the elongation of the stem is rapid it is continually stretched 

 beyond its limit of elasticity. This is shown by the fact that 

 such stems on wilting droop through several internodes domi- 

 nated by the collenchyma. We may therefore look upon the 

 collenchyma as a compromise between the need for strength 

 and the need to elongate during the growth in length. 



When growth in thickness sets in the collenchyma becomes 

 stretched tangentially and may even be broken apart in many 

 places (compare Figs. 23 and 24); but this is not apt to take 

 place until the bast fibers or the wood fibers also are ready to 

 reenforce it. Through all these vicissitudes of stretching and 

 tearing the collenchyma remains alive, till the end of the 

 season in annuals, and in perennials till the formation of a 

 deep-lying cork tissue shuts it off from the supply of water 

 and sap. 



In a very subordinate way the collenchyma may be used for 

 the slow conduction and temporary storage of materials, and 

 since it sometimes contains chloroplasts it may take part in 

 the manufacture of food (see Chapter IX). 



The Bast Fibers. As has been learned in Chapter II, the 

 bast fiber tissue may occur in the primary cortex, pericycle, or 

 secondary cortex. Where it occurs as a primary tissue in the 

 primary cortex or pericycle it is formed below the apex where 

 growth in length has ceased, following next to the collenchyma 

 in time (Fig. n). It may occur anywhere in the regions named, 

 from immediately beneath the epidermis to a position in front 

 of, and in contact with, the phloem portions of the vascular 



