THE TISSUES OF THE STEM 



37 



or milk- vessels and cells (Dandelion and Spurge). But however variable, it 

 is constantly present between the epidermis and the stele. 



The stele comprises the ring of vascular strands, together with the 

 tissues that envelop them, and the central column of pith. The 

 pith is a bulky mass of thin-walled parenchyma, its living 

 cells resembling those of the inner cortex. Similar tissue may 



FIG. 23. 



Transverse section of a vascular strand of Scrophularia nodosa. 6= endodermis. 

 />=pericycle. W=phloem. s= sieve-tubes. c=cambium. #y=xylem. p.v.= 

 pitted vessel. #=protoxylem. ( x 150.) 



extend between the vascular strands outwards to the endodermis. 

 A single strand of the stem of the Figwort thus isolated is shown in 

 transverse section in Fig. 23. It is typical of an herbaceous Dicoty- 

 ledon. The endodermis (e), indicated by its starch-grains, defines 

 the inner limit of the cortex. The vascular strand is roughly oval in 

 section, but it is not strictly circumscribed. It consists of two main 

 regions which differ in structure and in function. A softer region, 

 which lies peripherally, is called the phloem, or bast (phi) ; a firmer 

 region, which lies next the pith, is called the xylem or wood (xy). 



