Stem of A ngiopteris eveda. 517 
on the longitudinal radial walls. This structure has already 
been described by Mettenius 1 . 
In the large air-roots the outer elements only of the xylem 
become lignified ; the inner elements (Fig. 24 a) retain the 
form of tracheides, but their walls are not thickened in any 
way. In some cases the xylem of the air-roots was lignified 
half-way to the centre. 
In one air-root examined, certain of the rays of unlignified 
tracheides were not completed at their outer ends by groups 
of lignified elements, but instead of these, groups of phloem 
occurred. In one ray there lay successively phloem, a lig¬ 
nified tracheid, and unlignified xylem-elements. The non¬ 
completion of the ray of unlignified tracheides is of interest, 
since Koch states that in the normal development of the root 
these more central tracheides are the first to be differentiated 2 , 
while what is generally regarded as protoxylem is formed 
later at the outer end of the rays. These small elements 
at the outer end of the ray are certainly the first to be 
lignified, whether or not the more central unlignified elements 
in the shape of tracheides are formed later. 
The cortex in the root, as has already been described by 
Harting and Mettenius, is made up of two regions, the line 
of division between the two being particularly clear during 
the passage of the roots through the stem. The inner cells 
of the cortex bordering on the endodermis are thin-walled, 
rich in starch, and separated by intercellular spaces, which 
contain interwoven threads given off from the outer walls of 
the cells. These are the rodlets of Kuhn and others. The 
cells of the outer cortex have gelatinous walls, no cell-con¬ 
tents, and are separated by no intercellular spaces. 
The outermost cells of the cortex in the root are separated 
from the cortical cells of the stem by a layer of gelatinous 
substance, apparently similar in nature to that which con¬ 
stitutes the cell-walls in the outer cortex. In the older roots 
the distinction of the cortex into two layers is not so 
apparent, owing to the fact that the cells of the outer cortex 
1 Mettenius (’64), p. 518 . 2 Koch (’95), p. 377 . 
