98 THE PHYSIOLOGY OF PLANTS 



woody cells and vessels, and constitutes the wood or xylem ; 

 tte other contains the sieve-tubes, and is termed the bast or 

 phloem. The first-named portion conducts the raw solution 

 from the root to all the manufacturing cells, the second carries 

 away the organic material which has been built up in these 

 cells to all the places where new cells are to be formed. 



In the leafy expansions we see the conducting bundles 

 forming a delicate network, which penetrates the whole paren- 

 chymatous mass and collects in the midrib into the more 

 compact bundles. 



Each collection of bundles passes from the leaf into the 

 stem, runs down it through four or five stories, and then unites 

 with the bundles coming from a leaf at that lower level. In 

 this manner each leaf-trace bundle runs for a time between those 

 of the lower leaves, and at whatever point of the stem we 

 make a transverse section, we shall always find the conducting 

 systems of several superposed leaves. 



The bundles which come from the leaves appear as it were 

 let into the parenchymatous ground tissue, of which the first 

 year's axis is mainly built up. As the bundles of dicoty- 

 ledonous plants and of conifers are arranged in a ring (Fig. 

 1 2 A), they form later on a continuous ring which separates 

 a central tissue from the external regions. This central tissue 

 is termed the pith or medulla (M), the outer zone is called the 

 cortex (B). If the fibro-vascular bundles after their entrance 

 into the stem do not all run at a certain distance from the 

 periphery, but first bend in towards the centre of the stem -in 

 a gentle curve, and then bend back towards the periphery, a 

 transverse section would show no regular arrangement of the 

 bundles. It would reveal the bundles as dark, firm, irregularly 

 placed groups of tissue scattered through the ground tissue, 

 and not separating a pith from a cortical region. This is the 

 case in Monocotyledons, of which the stems of palms, Dracaenas, 

 and of the maize are easily procurable examples. 



§ 16. What is cambium ? 



But it is not only in the course of the bundles that we 

 notice a difference between Monocotyledons and Dicotyledons 



