7 o THE VEGETATIVE ORGANS 



layer of the cortex stands out clearly as a layer of larger 

 cells the endodermis {en). Examine the mature stem of 

 the Bean and compare it with that of the Deadnettle. 

 Although these stems are square, the general arrangement 

 of the tissues is the same as that which is found in the 



Buttercup. 



Scattered bundles of Monocotyledons. The stems of 



Monocotyledons differ in several 

 important respects from those just 

 described ; the vascular bundles are 

 scattered in the ground tissue (Fig. 

 35), and there is no cambium be- 

 tween the wood and bast, so that 

 when once these bundles are formed, 

 no further increase in thickness is 

 possible in them. They are therefore 

 called ' closed ' bundles to distin- 

 guish them from the ' open ' bundles 

 of Dicotyledons which possess a 

 cambium. 



Woody stems. Secondary growth. 

 The cambium of Dicotyledons, 

 though a small and inconspicuous 

 tissue, is a very important one, in- 

 asmuch as its cells are able to divide 

 repeatedly and form new tissue. 

 That formed on its outer side be- 

 comes part of the bast, while that 

 formed on its inner side adds to, and 

 increases the thickness of, the wood. 

 In a woody stem which lives for a number of years, cambium 

 is formed across each medullary ray, so as to form a cam- 

 bium ring (Fig. 36, i.f.c) . This later-formed cambium, called 

 interfascicular cambium, produces small new bundles 

 between the original large ones, as shown in Fig. 37. Growth 



Fig. 33. Diagram 

 showing the arrange- 

 MENT of Veins in 

 a Deadnettle Stem 

 (after Farmer). 



