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-layer. 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 lignified in a peculiar way (as for instance in 

 most thickish rhizomes, in the hollow scape of Allium, &c.). 



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 Draccena, Aloe, and Yucca, 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. — Nageli, Beitrage 

 zur wissensch. Bot. Heft. I. — Millardet, Memoires de la Soc. Imp. des Sci. Nat. de Cherbourg, 

 vol. XI, 1865. — [De Bary, Vergleichende Anatomic der Vegetationsorgane, 1877.] 



