770 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 



tissue while the outer cortex is to be regarded as a hypodermal layer, is shown not 

 merely by the characters of their respective cells, but by their relations to each 

 other in the neighbourhood of a stoma. This is seen in the transverse section on 

 PI. VI, fig. 34. The hypoderma, which in this case was clearly marked and con- 

 sisted of two layers of large cells (fig. 34, o.c), is not developed beneath the stoma 

 (st.), but the inner cortex (i.e.) here extends to the surface. Thus the well- 

 developed system of intercellular spaces in the inner cortex was placed in communi- 

 cation with the external atmosphere. In the longitudinal section shown in fig. 37 

 the inner cortex had disappeared, but the interruption in the hypoderma beneath 

 the stoma is beautifully shown. This, as fig. 34 showed, was originally filled by 

 cells of the inner cortex. 



While there is no doubt as to the hypodermal nature of the outer cortex in the 

 aerial stems, it must be borne in mind that a distinction of this narrow zone of 

 cells from the inner cortex is traceable throughout the plant. This holds even for 

 regions which there is no reason to think had a specialised hypoderma or assimi- 

 lating inner tissue. Thus it has been seen in the rhizomes (PI. IV, figs. 15 and 19), 

 and even in small axes without a stele (PI. X, fig. 73). 



The stem shown on PI. X, fig. 74, is peculiar in having one sector of the 

 transverse section composed of cells of larger size. This applies both to the 

 hypoderma and the inner cortex. 



Stele. — The stele throughout the plant is composed of a central strand of 

 tracheides surrounded by a zone of phloem. It is not delimited from the inner 

 cortex by any layers which can be interpreted as endodermis or pericycle. The 

 stele exhibits a considerable range in size, partly in relation to the region of the 

 plant, and partly in relation to the stoutness of the individual stems (PL V, figs. 21- 

 25 ; PL VII, figs. 44-46). Thus the stele represented in PL VII, fig. 45, is a fairly 

 large one, and has a strand of xylem composed of numerous tracheides ; while that 

 in fig. 46, which belongs to the slender stem represented in fig. 25, had only two 

 tracheides. Slender axes are also met with in which no vascular tissues have been 

 differentiated (PL VIII, fig. 60 ; PL X, fig. 73). 



The phloem in transverse section (PL VII, figs. 41 and 44) is composed of thin- 

 walled elements, four- to six-sided, and fitted closely together. No distinction can 

 be made between the various elements in this zone, which is about four or five 

 cells in depth. In longitudinal section (PL VII, fig. 42) the elements composing 

 the phloem are much longer than the cells of the inner cortex. They further differ 

 from the cortical cells in their end walls being oblique instead of transverse. As 

 comparison of figs. 42 and 43 will show, the elements forming the phloem become 

 shorter and less characteristic in the stele of the more slender upper region of the 

 stem. Although sieve plates have not been found, the position of this zone, its 

 clear appearance, and the form of its component elements seem to justify its 

 recognition as true phloem. 



