66 



BEGINNERS' BOTANY 



cells are thin-wallcd 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 

 libres 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, tlierefore, 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 

 bast contains (i) the sieve-tubes 

 (Fig. 8i) which transport the 



sap containing organic substances, as sugar 



and proteids, from the leaves to the parts 



needing it {s. Fig. 'j6\ 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 



fibres (Fig. 82) in the bast that serve 



for support. (3) There is also some 



parenchyma in the new Imst ; it is Fig. 82.— thick- 



c^ WALLED Bast 



now m part a stcrag3 tissue. bome- cells. 



Fig. 81. — Sieve-tubes, .f, .t, 



/ 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 

 by reagents. 



