20 STIGMARIA FICOIDES. 



whose broad parallel sides are parallel to the surface of the bark, whilst their shorter 

 axes are radial. In a word, the cells stand upon their thin bevelled edges, with 

 their two flat parallel surfaces severally directed towards the medulla and the 

 periphery of the bark." So far this description remains applicable to the corre- 

 sponding cells of Stigmaria. 



We learn something further respecting these curious cells from tangential 

 sections of them. We have such a section in Plate VIII, fig. 24. We now find 

 that the dark boundary lines, d", of Plate VIII, figs. 22 and 23, reappear as the 

 strongly marked walls of mother-cells, 24, d", whose vertical lengths rather exceed 

 that of their transverse ones, which explains the greater length of the parallel 

 tangential septa in the vertical section, Plate VI, fig. 9, d\ as contrasted with 

 their shorter lengths in figs. 22 and 23. But the tangential section further shows 

 that nearly every one of these tabular cells is undergoing secondary subdivisions 

 (fig. 24, d'", see also PI. V, fig. 49). Most of these secondary septa are horizontal 

 and parallel with one another, though occasionally a few vertical septa may also be 

 noted. 



I have dwelt with what may appear to be unnecessary minuteness upon the 

 structure of this cortical zone, but the results of these detailed studies are not 

 unimportant physiologically. We are dealing with anomalous primaeval morpho- 

 logies, and it is desirable to learn, so far as we can, the physiological truths which 

 these morphologies seem to reveal. The facts stated above demonstrate that in 

 the layer d' we have a peculiar meristem tissue or bark cambium of remarkable 

 activity, producing vertical cell divisions, seen in transverse and radial sections, as 

 well as horizontal divisions seen in tangential sections. Such meristemic action is 

 obviously designed to make additions to the cortical structures. The next question 

 requiring an answer is what part of the bark benefits by this meristemic activity ? 



One portion of this question is easily answered. Internal to the zone d we 

 have in all the more matured specimens a prosenchymatous zone, e, which, though 

 of very limited thickness in the younger roots, becomes the chief constituent of 

 the bark in older roots. In all the transverse sections the cells of this zone are 

 arranged in straight, radial, parallel lines, the individual cells becoming gradually 

 smaller and more prosenchymatous as they approach the interior of the root. 1 In 

 Plate VIII, fig. 22, e, they have a diameter of about x^oo ('00083) of an inch. At 

 fig. 23, e, of the same plate, their diameter is about -^q (*00125) to T'aVo ('00083). 



Plate IV, fig. 25, represents a transverse section, enlarged 3 diameters, of 

 a specimen of Stigmaria where this prosenchymatous zone, e, approaches near to 

 the wedges of the vascular cylinder, b ; and Plate VII, fig. 26, represents a portion 

 of the same zone further enlarged to 18 diameters. The thickness of the prosen- 

 chymatous layer in the original of these figures, now about '65, must once have 

 1 Tangential sections of these cells are seen in PI. VIII, fig. 24, a. 



