158 STRUCTURAL BOTANY 



the Wallflower. Starting from the side nearest the 

 middle of the stem, we first find annular or loosely- 

 coiled spiral vessels (often with a double spiral band), 

 then more closely-coiled spiral vessels of larger size, 

 and finally, next the phloem, vessels with simple pits. 

 We do not usually meet with bordered pits in the Lily. 



The phloem is of simple structure, and is entirely made 

 up of sieve-tubes and companion-cells, without any hard 

 bast or phloem-parenchyma. (Fig. 61, ph.) Thus both 

 wood and bast are composed of the same elements as 

 those of the Wallflower, but with rather less variety. 



The development of the vascular bundle is also such 

 as we are already familiar with. In the xylem the 

 first elements to be formed are those on the extreme 

 inner edge of the bundle (px in Figs. 60 and 61). As 

 usual, this protoxylem may be known by its annular 

 or loosely spiral vessels, adapted to undergo much 

 stretching after their walls are thickened. Conversely, 

 in the phloem, the first cells to become mature are 

 those on the extreme outer edge of the bundle. From 

 these two remote points the development advances 

 towards the middle. But now we come to a great 

 difference from our former type. In the Dicotyledon 

 we found that the development of the young bundle 

 into mature wood and bast was never complete ; there 

 was always a band of actively dividing cells left 

 between the two, so that the formation of new wood 

 and bast could go on without limit. In other words, 

 the bundle possessed a cambium, and was therefore 

 said to be open. 



In the Lily and all Monocotyledons, on the other 



