THE TISSUES OF THE STEM 



45 



each of the largest strands on entering the stem from the leaf slants 

 sharply inwards, but short of the centre it curves again outwards, 

 and gradually approaches the periphery. There it fuses with other 

 strands. As the strand is thickest in the middle of this course, the 

 consequence is that the strands are fewest and largest at the centre, 

 and smaller but more numerous near to the periphery {Fig. 31). 



Fig. 32. 



Transverse section of a vascular strand from the internode of Zea.Mais. o= annular 

 tracheid. s^ = spiral tracheid. w, w' = vessels with bordered pits. i; = sieve- tubes. 

 s= companion cells. c^r= compressed first tissues of phloem. / = intercellular space. 

 vg= sclerotic sheath. /= conjunctive parenchyma. ( x i8o.) S. 



Thus the difference between the arrangement of the strands in 

 Dicotyledons and Monocotyledons is not fundamental. The one type 

 graduates by intermediate steps into the other. 



Similarly the structure of the strand itself is on the same plan in 

 both, the most conspicuous difference being the absence of cambium 

 in the Monocotyledons. One of the larger strands towards the centre 

 of the stem of Maize shows features usual in them. (Fig. 32). It is 

 embedded in thin-walled conjunctive parenchyma of the stele, and is 



