R. N. Salaman and J. W. Lesley 25 



Occasionally a slender band of thick-walled lignified elements is seen 

 joining two of the three main bundles, sometimes two or even three thin 

 bands of wood may be found at certain levels so that in the latter case 

 a feeble but continuous ring of wood is formed at that level, but this 

 condition is exceptional. Naturally enough the interfascicular cambium 

 too is imperfectly developed. As a rule it extends along one or two 

 sides of the stem and forms secondary xylem and phloem. Or, again, 

 the interfascicular cambium may show signs of division only at certain 

 points. The phloem is either scanty or only formed at intervals between 

 the bundles, but wherever there is cambium phloem is found generally 

 forming isolated groups according to the disposition of the cambium. 

 The deficiency of phloem is less marked than that of xylem. Indeed it 

 is the incomplete development of thick-walled lignified interfascicular 

 xylem which is the characteristic anatomical feature of the " prostrate " 

 plant. 



The other tissues of the " prostrate " stem are the same as those of 

 the normal type already described. The collenchyma is present in the 

 outer cortex, only the larger cortical cells in the interfascicular region 

 being somewhat rounder and less elongated tangentially owing to the 

 lack of pressure from within which is exercised where much inter- 

 fascicular xylem is formed. The internal phloem is fully developed, 

 thus confirming its independence of the cambium. 



The " prostrate " stem is more definitely triangular in shape than the 

 upright, owing to the deficiency of interfascicular secondary growth. 



The " prostrate " habit of growth is therefore associated with a de- 

 ficiency of mechanical tissue which impairs its rigidity. For whilst the 

 " upright " stem is practically a tube and owes its rigidity to a continuous 

 cylinder of wood, the " prostrate " consists of a few strands connected, it 

 is true, at certain points, but for the most part effectually isolated. 



When once the pure breeding " prostrate " variety had been recog- 

 nized, it became obvious that in our cultures certain plants we had met 

 and which had been taken to be a " prostrate " variety were, though very 

 similar in habit, not to be confounded with the true " prostrates " already 

 described. Some of these plants and those which were subsequently 

 extracted in pure culture at Cambridge resemble the " prostrate " in the 

 lack of rigidity and rubber-like feel of their stems — a resemblance which 

 is borne out in detail by their histological structure. They all differ, 

 however, in one striking characteristic from the " prostrate " variety. 

 The stems, once fallen to the ground, do not creep along horizontally 

 as in the first variety, but after a short time the distal portions turn up 



