348 PRIMARV ARRANGEMENT OF TISSUES. 



the endodei-mis borders externally. The xylem consists in the middle of wide scalari- 

 form tracheides, and interstitial bands of Parenchyma. According to all the data, which 

 however require to be more exactly established, the bundles described may represent 

 an intermediate form between the concentric and radial Fern-bundles. According to 

 Russow the likewise axial bundle of the stolons of N. pectinata and rufescens has 

 not the structure described. 



3. Radial Bundles. 



Sect. io'/. The radial bundles are closely connected with the concentric by 

 means of those in the stem of Lycopodium, and by the diarch forms, which occur in 

 many roots. In the" typical cases they are distinguished by the fact that in the radial 

 bundle the xylem forms several bands running out radially from the centre, between 

 which lie the same number of groups or bands of phloem alternating with them. In 

 all radial bundles the development of the characteristic elements, both of the xylem 

 and phloem bands, begins at the periphery, and proceeds thence with varying 

 celerity towards the middle. The primitive elements, which in the xylem are here 

 also distinguished by^ their narrowness, form the peripheral ends of the rays. As 

 these thus form the points of departure of the development of xylem, it is usual 

 to speak of the number not of the rays, but of the starting-points — of di- to 

 polyarch bundles ^- 



Radial bundles occur in the stems of Lycopodiaceae, and in the filiform stolons 

 of Nephrolepis; and in all roots, with a few exceptions mentioned at p. 319. 



The axial strand which traverses the middle of the stem of Lycopodiaceae agrees, 

 with the exception of its radial structure, with the bundles above described in the 

 stem of the Selaginellae (with the exception of S. spinulosa), which, in the struc- 

 ture and development of their xylem, correspond to the diarch or oligarch radial 

 forms. 



In the stem of Psiloium ^ this strand is cauline, not receiving or giving off leaf- 

 strands. In the branches which appear above ground the cross-section of the whole 

 is almost circular, bounded on the outside by an endodermis, which only differs from 

 the surrounding parenchyma in its undulating radial walls. The xylem is triarch to 

 pentarch and octaich ; its not always equidistant rays are separated from the endo- 

 dermis by one or a few layers of relatively narrow, elongated, prismatic parenchymatous 

 cells, and consist at their peripheral ends of a group of narrower reticulated tracheides 

 (I did not find spiral tracheides), and towards the centre of a few rows of scalariform 

 tracheides ; these rows do not reach to the middle of the bundle, but abut on a strand 

 of elongated prismatic, pointed sclerenchymatous fibres, which traverse the middle. 

 The rest of the substance of the bundle consists of thin-walled prismatic parenchyma, 

 in which, especially at its periphery, are scattered some few-celled groups of somewhat 

 narrower and thicker-walled sieve-tubes. This designation is at least justified by the 

 appearance of the smooth lateral walls, agreeing with that in the Ferns, by the 

 granular contents adhering obstinately to the walls,' and by the absence of nuclei, 

 which is very conspicuous on comparing these elements with the surrounding cells ; 

 on the thin oblique terminal-surfaces of the articulations I, believe that I have directly 



' Nageli, Beitr. /. c. p. 10. ^ jj^geli^ /. c._R„ssow, /.<:. p. 131. 



