MONOCOTYLEDONS . 
629 
With respect to their Histology^, Monocotyledons differ from Dicotyledons and 
Gymnosperms chiefly in the course of the fibro-vascular bundles in the stem, and in 
the want of a true cambium-la)'^r. A number of common bundles (/. e. those com- 
mon to the stem and leaves) enter the stem side by side from the broad insertions 
of the leaves, pass obliquely downwards into it, and then again bend outwards as 
they descend, approaching gradually the surface of the stem. The common bundle 
is usually thickest and most perfectly developed at the curved portion which lies 
deepest in the stem, while the limb which bends upwards into the leaf becomes thinner 
and simpler upwards, and the descending limb of the bundle behaves similarly down- 
wards. Hence a transverse section of the stem which cuts through the different 
descending limbs at different heights in their course shows bundles of different structure 
and of various sizes. A radial longitudinal section through the bud or through mature 
stems with short internodes (as Palm-stems, thick rhizomes, bulbs, &c.) shows how the 
bundles which descend from different leaves, the curves of which lie at different 
heights, cross one another radially, some of them bending inwards where others are 
already turning outwards. In elongated internodes, as for instance those of the stalks 
of Grasses and of some Palm-stems (like Calamus), the long scapes of Allium, &c., the 
bundles run nearly parallel to one another and to the surface ; the curves and inter- 
sections of the bundles may be easily distinguished at the apex of such stems, and localise 
themselves in the transverse plates or nodes which do not elongate between each pair 
of internodes. The nodes are not unfrequently traversed by a network of horizontal 
bundles ; this is very conspicuous in the Maize. 
The course of the fibro-vascular bundles which has now been described renders 
impossible the separation of the fundamental tissue of the stem into pith and cortex 
in the sense in which this occurs in Conifers and Dicotyledons. The parenchymatous 
fundamental tissue fills up homogeneously the spaces between the bundles which are 
generally numerous ; but a separation takes place not unfrequently into an outer peri- 
pheral layer and an inner region, a layer of tissue being formed between the two 
the cells of which are thickened and hgnified in a peculiar way (as for instance in 
most thickish rhizomes, in the hollow scape of Allium, Sec). 
In consequence of their not being parallel, and of their scattered distribution in the 
transverse section of the stem, the descending bundles of Monocotyledons have not the 
power of coalescing into a closed sheath by connecting bands of cambium (interfasci- 
cular cambium), as is the case in other Phanerogams. In correlation with this the layer 
of cambium between the phloem and xylem is also absent ; the fibro-vascular bundles 
are closed. When a portion of the stem ceases to grow in length, the whole of the 
tissue of the bundles becomes transformed into permanent tissue (see e.g. Fig. 92, 
p. no) ; and there is in consequence usually no subsequent increase in thickness; each 
portion of the stem, when once formed, maintains the thickness which it had already 
attained within the bud near the apex of the stem. But in Dracana, Aloe, and Tucca, a 
renewed increase of thickness begins afterwards at a considerable distance from the 
apex of the stem, which may even continue for centuries and may cause a considerable 
though slow increase in its circumference. But this subsequent growth in thickness 
takes place in a way quite different from that which occurs in Gymnosperms and Dico- 
tyledons ; — a layer of the fundamental tissue parallel to the surface of the stem becomes 
transformed into meristem which continually produces new closed fibro-vascular bundles, 
and between them parenchymatous fundamental tissue (Fig. 104). A more or less 
evidently stratified network of slender anastomosing bundles is thus formed, the posi- 
tion and connection of which is easily recognised on stems which have been exposed 
to the weather, and in which the parenchyma which fills up the interstices has 
^ Von Mohl, Bau des Palmenstammes, in his Vermischte Schriften, p. 129. — Nägeli, Beiträge 
zur wissensch. Bot. Heft. I. — Millardet, Memoires de la Soc. Imp. des Sei. Nat. de Cherbourg, 
vol. XI, 1865 — \J^^ Bary, Vergleichende Anatomie der Vegetationsorgane, 1877.] 
