66 



PLANT BIOLOGY 



cells are thin-walled and filled with protoplasm. During 

 the growing season they are continually adding to the 



wood within and the bark with- 

 out ; hence the layer moves out- 

 ward as it deposits the new 

 woody layer within. 



The bark consists of inner or 

 fibrous bark or new bast (these 

 fibers in flax become linen), the 

 green or middle bark which func- 

 tions somewhat as the leaves, 

 and the corky or outer bark. 

 The common word " bark " is 

 seen therefore not to represent 

 a homogeneous or simple struc- 

 ture, but rather a collection of 

 several kinds of tissue, all sepa- 

 rating from the wood beneath 

 by means of cambium. The new 



Fig. 8i. — Sieve-tubes, s,s; 



p shows a top view of a sieve-plate, 

 with a companion cell, c, at the 

 side; o shows sieve-plates in the 

 side of the cell. In s, s the proto- 

 plasm is shrunken from the walls bast COntaillS (i) the sicVC-tubcS 



byreagents ' • (Fig. 81) which transport the 



sap containing organic substances, as sugar 

 and proteids, from the leaves to the parts 

 needing it (s, Fig. 76). These tubes have 

 been formed like the wood vessels, but 

 they have sieve-plates to allow the dense 

 organic-laden sap to pass with sufficient 

 readiness for purposes of rapid distribu- 

 tion. (2) There are also thick-walled bast 

 fibers (Fig. 82) in the bast that serve for 

 support. (3) There is also some paren- 

 chyma (parent tissue) in the new bast; 

 it is now in part a storage tissue. Some- 



Fig. 82.— Thick- 

 walled Bast 

 Cells. 



