330 THE BIOLOGY OF THE PLANT 



Do they resemble the latter in structure ? Is the 

 sclerenchyma arranged as in the rhizome ? Do 

 the sclerenchymatous elements have the same 

 shape, size, contents, etc., as in the latter? Com- 

 pare the parenchyma in the two parts. Study 

 carefully the fibro-vascular bundle. Note 

 the position and shape. Study the bundle- 

 sheath. Of what kind of cells is it composed ? 

 Of how many layers of cells is it formed ? What 

 is the structure of their walls? What do the 

 cells contain ? Do you find intercellular spaces 

 present? Within the bundle-sheath find the 

 phloem-sheath, consisting of parenchymatous 

 cells containing starch. What is the shape of 

 this sheath ? Does it everywhere have the same 

 thickness? Do the cells contain protoplasm? 

 Inside the phloem-sheath comes the bast or 

 phloem, the outer portion of which consists 

 of thick-walled elements, the protophloem. 

 What is the shape of the cavities in this layer ? 

 Are there any intercellular spaces ? Next with- 

 in this comes the true phloem, consisting partly 

 of elongated, thin-walled elements, the sieve- 

 tubes, and partly of parenchymatous cells, the 

 bast parenchyma. How many rows of sieve- 

 tubes are there ? What is their shape in cross- 

 section ? What do they contain ? What is their 

 position in regard to the bast parenchyma? The 

 centre of the bundle is occupied by the xylem 

 or wood, consisting mainly of thick-walled ele- 

 ments. Note the polygonal shape of some of 

 the xylem elements, the tracheids, when seen 

 in cross-section. How are these elements ar- 

 ranged with regard to one another? What do 



