480 
PEOFESSOE HENFEET OX THE AXATO^^IT OF YICTOEIA EEGLA. 
ent from those of the layers of Dicotyledons, being formed of entirely ne"^ fibro-rascular 
cords quite independent of those of the inner substance of the stem, while the successive 
layers of the stems of Dicotyledons are ordinarily horizontal developments from the 
cambium layers of the pre-existing bundles. 
Many Dicotyledonous rhizomes, although perennial, form no distinct annual rings, and 
it is common to find the older parts of these thinner than the younger ; the reverse of 
what is ordinarily seen in trees of this class. But in those cases even where there are 
no annual rings, it is almost always possible to distinguish the t}q)ical aiTangement of 
the bundles of the yearling shoot in a single circle surrounding the pith, uith processes 
of cellular tissue forming medullary rays, running out between them to the lind or 
bark. This circular arrangement is seen most distinctly when the stem has the inter- 
nodes developed, but it is evident even when they are little elongated. The arrange- 
ment of the fibro-vascular bundles in a single circle depends upon the fact, that the 
leaves of Dicotyledons are supplied by a comparatively small number (3— -5 — 7) of 
fibro-vascular bundles which are developed collaterally, and in their course through the 
few internodes which they traverse, remain side by side, separated by plates of cellular 
tissue (medullary rays) which are mterrupted at more or less distant intervals by colla- 
teral anastomoses of the bundles. The bundles supplying the leaves are formed in the 
bud, and their earliest vessels constitute the sheath of the central pith ; as the leaf is 
carried out from the ^unctmn vegetationis, no new bundles are developed, but the fibro- 
A'ascular bundles are increased in diameter by the development of wood-cells and ducts 
at their peripheral surface, and in most cases at the sides, thus encroaching on the 
medullary rays. 
In Monocotyledons it is usual to find the nascent leaf embracing with its base a 
much greater segment of the circumference of the growing apex of the stem than in 
Dicotyledons ; often it extends all round, so as to appear like, or actually to form a tube, 
as we see even in the full-grown state of the Grasses and Sedges. Into this wide base 
run generally a great number of separate, small, fibro-vascular bundles, as is seen very 
clearly in broad parallel-veined leaves, such as those of the Flag. And when a petiole 
is formed, the bundles do not unite collaterally as in the petioles of Dicotyledons, but 
pass out separately, as they run in the stem. Again, as the leaf is pushed ouBvard in 
the bud, towards the circumference of the stem, the earlier bundles receive no peri- 
pheral additions, but in place of this neAv and distinct bundles are formed on the outer 
side of the old ones. Further, the new bundles formed for the succeeding leaves are 
not placed between these older ones, but are formed within and cross to the outside of 
them, passing down in the peripheral region; those of the next leaf follow in the 
same way ; and when, as is often the case, the bundles traverse a great number of inter- 
nodes (20 — 50), a vast number of them must necessarily appear in a cross section of the 
inferior part of the stem. These bundles contain no provision for enlargement in sub- 
sequent seasons of growth : the delicate elongated cells called by Vox Mohl vasa propria, 
and compared by him with the cambium structure of Dicotyledonous bundles, are 
