FROM THE LOWER COAL-MEASURES. 89 



instances, however, the diameter of the tracheae is nearly uniform. 

 The average diameter is about 0-04 milliro., but some reach 

 0-061 millim., while others are not more than 0014 millim. 

 "Where a radial row contains tracheae of different sizes, the chai w 

 is either abrupt or gradual, and when a certain maximum has 

 been attained there is no further increase. The number of 

 tracheae in each row is seldom above 6 or 7, and may be as low 

 as 3 or 4, and in several instances the outermost have their 

 walls still unthickened, showing that the order of development 

 has been centrifugal. In some of the bundles the rows of 

 trachea are in close contact laterally, and no xylein parenchyma 

 appears to be intercalated between them. In others the trans- 

 verse sections show the rows of tracheae separated by what looks 

 like thin-walled parenchyma, but this is not found in the longi- 

 tudinal sections. In the radial direction the tracheae appear to 

 be united directly in all cases. In no part of the xylem has any 

 trace been found of sclerenchymatous fibres. 



Procambium or Cambium.— Outside the xylem of each bundle 

 is a thin layer of cells (PI. XVII. fig. 3, c), small in size and with 

 delicate walls, which form a tissue that is either of the nature of 

 fascicular cambium, or is part of the procambium string which 

 has not been converted into xylem and phloem. Against the 

 former alternative is the fact that the elements do not show a 

 radial arrangement, although in one or two of the bundles there 

 is an approximation to this. Interfascicular cambium is also 

 altogether absent. On the whole, the evidence is against the 

 view that this is a cambium layer, and leans rather to the second 

 alternative of procambium. This, however, docs not preclude 

 the possibility of its subsequent conversion into cambium. 



Phloem.— A narrow zone of tissue, averaging 0-169 mill.m. in 

 breadth, lies outside the layer just described (fig. 3, ph). In 

 most of the sections it has undergone much disorganization, and 

 no histological elements are recognizable in it. In two of them, 

 however, it is better preserved, and these show that it is inter- 

 rupted opposite the medullary rays, and really consists of as 

 many separate masses as there are masses of xylem. This feature 

 and the position the layer occupies seem to show that it repre- 

 sents the phloem part of the vascular bundles. Its elements, 

 where preserved, are seen to be mostly thin-walled, but some of 

 them have the appearance of young sclerenchymatous fibres, 

 especially towards the outer periphery. As, however, they are 



