INTRODUCTION ii 



the stem. The sieve tubes promote the upward 

 transport of the starch, proteids, etc. They are 

 associated with companion cells. The sieve paren- 

 chyma cells transport the carbohydrates and store 

 up proteids, transferred from the sieve tubes, and 

 carry into the medullary rays this material, for 

 retention or further transference to the xylem or 

 wood for transport, or to the parenchyma for storage. 



The wood consists of tracheal or water tubes, 

 tracheids, xylem parenchyma, and wood fibres. The 

 tracheal tubes, where thick, serve to strengthen, the 

 thin tubes serving for the passage of water and 

 nutrient solutes. They thus function as water- 

 carriers from the roots to the leaves. The tracheids 

 also serve as strengthening tissues and for conduct- 

 ing water. The xylem parenchyma stores up a 

 reserve of water and food, and may also transfer 

 water upward to the leaves. The wood fibres are 

 designed to impart strength almost entirely, and are 

 not so sappy as the phloem. 



The parenchyma cells store up starch. The 

 cambium, lying between the phloem and xylem 

 external to the latter, occurs in Dicotyledons, and 

 serves to increase the thickness of the vascular 

 bundles by the formation of secondary xylem and 

 phloem. 



The medulla or pith, which is central, serves 

 temporarily for the storing and conduction of water, 

 and also sometimes of food materials. 



The intermediate primary meristem between the 



