14 



THE PHYSIOLOGY OF PLANTS 



outside. Tlie chief conducting apparatus for the food material 

 which has been absorbed by the root-hairs and passed on 

 through the cortical tissue is the central cylinder, which con- 

 tains wood vessels, indicated in Fig. i G^ by narrow rows of 

 cells. These vessels, which appear in Figs. 3 and 4 as darkly 

 outlined spaces, do not lie singly in the transverse sections, 

 but are collected in groups. In the root of the Shallot, as in 

 many other Monocotyledons, the groups of vessels are arranged 



Fra. 3.— The Central Vascular Cylinder op the Root op the Shallot 

 (Allium ascalonwum). 

 U is the very wide central vessel which joins the five dark rays of smaller wood vessels ; 

 p is the layer of cells which gives rise to new lateral roots (pericambium) ; s, the endo- 

 dermis or bundle-sheath ; d, the thin passage cells (after Haberlandt). 



in rays, or like the spokes of a wheel round a central very 

 large vessel (Fig. 3 g), so that we seem to have a single group 

 of wood vessels running out into five arms or plates (Figs. 3, 4). 

 In the root of the Auricula (Fig. 4) the seven separate groups 

 of wood vessels (g) are arranged in a ring round a soft par- 

 enchymatous central tissue, which is termed pith or medulla. 

 In the Dicotyledons this pith often replaces the large central 

 vessel of the Monocotyledons, and the root structure resembles 

 more closely the structure of the stem. We need only imagine 



