Tissues in Certain Monocotyledons. 53 
The cortex and epidermis are already nearly fully formed, 
and in the former the tannin-sacs have acquired their charac- 
teristic contents. The stage shown in Fig. 1 1 is at about 
1 mm. from the apex. At this distance the bundles of the 
cylinder have become more straightened, owing to the 
elongation of the internodes, and so better sections can be 
obtained. Otherwise there is not much change. The 
divisions at the periphery of the cylinder (Sanio’s ‘ Thickening 
Ring ’) take place irregularly in all directions. 
At 5 mm. from the apex considerable progress has been 
made. The primary development of the cylinder as a whole 
has almost ceased. Scarcely any fresh divisions are now 
found in the ‘ Thickening Ring.’ 
Many of the outermost bundles of the cylinder, however, are 
still in an early procambial stage. The principal bundles appear 
to have their phloem fully formed. The proximal part of their 
xylem has been completed in the usual centrifugal order. 
A few of the elements of the distal half of the xylem- 
ring are also becoming lignified. In this part of the bundle 
the differentiation of the xylem-elements follows no definite 
order. We may speak of this later-formed, non-centrifugal 
part of the xylem as metaxylem \ In the large bundles the 
phloem follows the normal centripetal order of differentiation. 
In all the other bundles (forming the great majority) the 
whole of the xylem is metaxylem. There are no spiral 
elements, and there is no centrifugal development. Dif- 
ferentiation begins indiscriminately at any points of the 
xylem-ring, and no preference whatever is shown for its 
proximal side. So too with the phloem ; in these later- 
developed bundles the phloem does not develope centri- 
petally ; so far as any regular order can be traced, the phloem- 
elements in the middle of the bundle appear to be completed 
first. 
1 The term was introduced by Van Tieghem for that part of the primary xylem 
in the root which is differentiated after the normal centripetal development is 
completed ; see his Traite de Botanique, 2nd ed. p. 684. Mutatis mutandis the 
same term may well be applied to late-formed non-centrifugal xylem in a bundle 
belonging to the stem. 
