PRIMARY STRUCTURE OF CERTAIN PALEOZOIC STEMS. 



347 



more the appearance of having been caused by unequal contraction of the tissue, per- 

 haps after death.* Where the specimen is badly preserved, the gaps are much ex- 

 aggerated. The pith has evidently undergone dilatation, as shown by the increase in 

 width of the medullary rays at their inner ends, and by the marked horizontal elonga- 

 tion of many of the pith-cells (PL II. phots. 8 and 9, PL V. figs. 14-16). The latter 

 feature is especially conspicuous around the primary xylem-strands, from which the 

 dilated medullary cells usually radiate out in all directions (phots. 8 and 9, figs. 14 

 and 15). This is a familiar phenomenon wherever lignified strands occur in the midst 

 of an actively-growing cellular tissue, as, for example, in fleshy roots. 



The chief point of interest in the stem of Pitys antiqua is the presence of numerous 

 strands of primary xylem around the pith, and for the most part embedded in its tissue. 

 Their distribution is shown in diagram 5, prepared by Mr L. A. Boodle, in which all 



2 ' *J 



the strands shown in a transverse section have been accurately plotted, in their exact 

 position, with the aid of the camera lucida. The total number of xylem-strands present 

 in this section was 46. It will be noticed that, with few exceptions, they are separated 

 by an appreciable interval from the inner edge of the secondary wood. Actual contact 

 is only shown at three points, namely, in the case of the strands numbered 13, 23, 40. 

 All the others are separated from the wood by distances ranging from *3 mm. to 1*8 

 mm.f The xylem-strands themselves vary in diameter from about *15 mm. to about *3 

 mm., so their distance from the wood almost always exceeds their own diameter, and is 

 often many times as great. Strands 13 and 40 are double (cf. PL V. fig. 14 for strand 

 13) ; we know, from the evidence of successive transverse sections, that strand 13 passed 



* Similar lacunae are present in the central tissue (primary wood) of Megaloxylon. See Seward, " Notes on the 

 Binney Collection of Coal-Measure Plants " ; Part ii., Megaloxylon. Proc. Cambridge Phil. Soc, vol. x., 1899. 



t Of course only that part of the section in which the secondary wood is present is taken into consideration. 



TEANS. ROY. SOC. EDIN, VOL. XL. PART II. (NO. 17). 3 f 



