THE STEM. 



to a type of tissue known as sclerenchyma, characterized by thick- 

 ened woody cell- walls and absence of protoplasm. In the soft bast 

 there is nothing very 

 remarkable about 

 the parenchyma, but 

 the sieve tubes are 

 very important. 

 They are the essen- 

 tial part of the bast, 

 and are always pre- 

 sent in it, at any 

 rate in phanero- 

 gams. The radial 

 section will show, if 

 carefully prepared, 

 that each sieve tube 

 is made up of a 

 series of elongated 

 members or joints, 

 somewhat swollen at 

 their ends, and sejia- 

 rated by transverse 

 partitions, the sieve 

 plates perforated (as 

 we have seen in the 

 cross - section) by 

 numerous pores. 

 We have again to 

 deal, in all proba- 

 bility, with cell-deri- 

 vates. Each mem- 

 ber was originally one of a row of cells. The walls of the 

 mature tubes, although thickened, remain of cellulose, but the 

 protoplasm seems to have disappeared. Each member is lined 

 by a slimy substance, which is denser, and therefore stains 

 more deeply than the rest. The transverse party- walls between 

 the original row of cells appear to have been thickened, pits, 

 however, having been left, which corresponded on opposite 

 sides. Hence at any spot where these occurred there would be 

 a pit or depression on either side, the two being separated by 

 the pit-membrane, absorption of which would lead to formation 

 of a little canal, piercing the sieve plate. In the sieve tube the 

 slime in one member is connected with that in the adjoining ones 

 by threads of the same material running through the sieve plates. 

 We have here an example of a cell-fusion or vessel, which terms 



c 



Fig. 6.— Transverse and Kadial Sections of Sunflower Stem, to 

 show structure of a Vascular Bundle. [After Prantl.] Much 

 magnified. B.*h. bundle sheath ; P. pith ; co. cortex ; X. 

 xylera ; 88. spiral vessels ; tt. pitted vessels ; x.f. xyleni 

 fibres ; Ph. phloem ; s.t. sieve tubes ; phf. phloem fibres; 

 C.f, C.i. cambium. 



