440 DR. ERIC DRABBLE ON THE 
Sometimes the elements are square-ended and shorter, being of a more definitely 
selerenchymatous nature. 
The fibres arise in the procambial strands, or procambial cylinder, as the case may be, 
at the apex of the root, and form the ground-tissue of these structures. The elements 
are at first small, somewhat elongated and square-ended, subsequent growth resulting in 
the fibrous nature of this tissue. 
Lignification sets in in a number of circles, each surrounding a large internal vessel of 
this zone, and proceeds towards the vessel from all sides; often even in a very large and 
mature root one or more layers of cells immediately surrounding the vessel may be 
non-lignified. 
The protoxylem elements also are frequently embedded in a cellulose-walled group of 
cells. | 
The Vessels. 
The vessels of the xylem occur in radial groups with exarch protoxylem, having the 
normal arrangement for monocotyledonous roots. These are very often accompanied 
internally by large vessels scattered throughout the fibrous zone. 
The external elements of the protoxylem are thickened in an annular manner (PI. 49. 
fig. 46) and are generally lignified early. The vessels of the metaxylem are scalari- 
formly thickened, as are also the large internally scattered vessels. These latter are 
frequently quite without lignification, even in the mature root, the wall giving a very 
different cellulose reaction with iodine and sulphuric acid. 
The arrangement of the scalariformly marked areas is of the nature commonly met 
with in large roots of Monocotyledons and in large tracheids of ferns, but is here more 
regular than usual (Pl. 49. fig. 49). 
In the large vessels the transverse walls marking the originally distinct elements from 
which the vessel is built up often partially persist, and in any case the vessel is constricted 
at these points, the whole structure when separated out by Schultze’s macerating-fluid 
presenting a moniliform appearance. 
The internal vessels are often very large, particularly in Corypha umbraculifera 
(Pl. 50. fig. 70). 
In some cases they are closely packed and lateral pressure has led them to adopt a 
radially elliptic form instead of the usual circular outline. 
The elements immediately surrounding one of these large vessels are sometimes 
marked in a scalariform manner in the same way as the vessel itself; these elements 
are expanded at the points of constriction of the vessel and fill the depression (fig. 49). 
A mass of deeply staining mucilaginous substance is often found in the vessels in the 
region of the partially disintegrated transverse walls. 
In certain cases the transverse walls may be thickened and lignified in bands 
stretching across the end-plate; the rest of the end-plate may then disappear, leaving 
trabeeulw of lignified material extending across the cavity of the vessel. 
. Thylosis-formation is of very frequent occurrence in the larger vessels. This is 
perna well Ee in Areca concinna (Pl. 49, fig. 51) and Cocos flexuosa (Pl. 49. 
Geet 
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