212 Barratt. — A Contribution to our Knowledge of the 
former records examples of certain irregularities in the direction of its 
lignification. 
With regard to the elements composing this metaxylem, Queva (7), 
from an examination of E. maximum and E. limosum , considers that they 
are all spirally thickened tracheides ; but Eames (10) mentions that in 
E. maximum the elements of the lateral strands generally show sclariform, 
or reticulately pitted walls with occasional annular and spiral thickenings. 
According to the results of the present investigation, there is no 
question that in E. arvense the majority of the tracheides composing this 
metaxylem are spirally thickened, the first developed ones being consider- 
ably drawn out during the later stages of elongation, and in some cases the 
elements break down’ leaving small lacunae comparable to those associated 
with the protoxylem. On the other hand, in the later-formed elements it is 
seen that the thickening is more closely disposed until a simple form of 
reticulum results, requiring careful examination to distinguish it from the 
closer spiral forms. 
This is found most elaborately developed in E. giganteum , and con- 
firms Gwynne- Vaughan’s observation that the smaller size of the outer 
elements suggested a centripetal order of development. 
The order of differentiation of this particular region of the vascular 
system has been carefully traced both in the young sporeling and in the 
apices of adult shoots belonging to the following species : E. arvense and 
E. maximum / Although there is a variation in the amount of xylem 
developed, the order of its appearance is fairly constant. 
The tissues which are destined to form these lateral xylem elements 
are early distinguishable as procambial strands. Elongation of the elements 
is almost completed in the internodes before any thickening takes place, 
although in some cases where growth has continued after the completion of 
the tracheides a certain amount of disruption occurs with the formation of 
small lateral lacunae. The thickening and lignification of the tracheides 
proceeds regularly through the internode ; beginning in one cell which 
abuts upon the nodal tracheides, the process follows in the cells immediately 
below until a whole column is lignified (Text-fig. 8 and PI. VII, Fig. i). Before 
this is completed, however, another vertical row of cells begins to differentiate 
in the same way, and this scheme is followed until the whole group of 
tracheides is matured. Owing to the fact that the lignification does not take 
place simultaneously throughout the internode, it is possible to select one in 
which the material is in a half-developed condition. Some stem apices about 
two inches long were halved longitudinally, and one half of each apex was 
cleared with eau de Javelle and stained with fuchsin in the way already 
described. From these it could be ascertained which nodes showed the 
desired condition, and the companion halves were embedded in wax and 
microtomed. From the two sets of preparations it was possible to 
