Stem

Straight, cylindrical, monopodial, single without building stolons at the base; smooth, grey, the stem is not considered bole. The palm reaches heights between 5 to 20 m and diameters between 8 and 30 cm (CARVALHO 1993). At the apex, place of leaf insertion, there is a smooth, green-olive to dark-green section of 1.0-1.5 m, thicker than the trunk, formed by a bunch of leaves. Within this section is the edible part of the palm.  

Stem of a circ. 40 year old E. edulis plant

Wood

Like the other Monocotyledons, the palm does not have a vascular cambium (meristem tissue layer between wood and bast that produces new cells), which causes the typical secondary growth. Thus, the growth in thickness is merely based on the expansion of existing cells. In the strict sense, if hereby by the cambium inwards secluded permanent tissue of Gymnospermae and Dikotyledonen is designated, palms do not have wood. But in its technological features and terms of lignification of the cell structures, the palm tissue is comparable with wood and will therefore be characterized as such (GROSSER 1977).
In contrast to the wood of conifers and hardwoods, the inside stem region of palm trees is always brighter than the outside. This difference is caused by groups of shaded cells, the so called vascular bundles, visible mainly in cross section. These bundles are immersed in a clear thin-walled tissue (parenchyma) and give the stem a great rigidity and stability. Due to the columnar growth of the stem, the palm wood is not homogeneous, but on the static point of view more solid on the outside and softer on the inside. Like a tube the soft inner core is surrounded by a hard outer sheath. As their main vascular system is not located in the outer area of the stem, the palm is excellently protected against fires.

Anatomical structure of the palm wood:

Macroscopically

(visible with the naked eye or at least magnifier)

Macroscopic magnification of vascular bundles. Left: wide vascular bundle of the outer stem area with a high proportion of thick-walled, dark brown fibre cells as well as one or two Metaxylems. Right: small vascular bundles of the inner stem area, few in number and embedded in the bright parenchymatous ground tissue

Visible in the cross-section of a palm stem are numerous scattered arranged black dots, the vascular bundles. Their number and dimension is decreasing strongly towards the stem centre. Beside this the brighter parenchymatous ground tissue and the small, a few millimetres thick bark is well-defined.

Typical E. edulis plant with infructescences

Stems of circ. 8-15 year old plants of E. edulis

Vascular bundles in the wood of a circ. 40 year old E. edulis plant

Microscopically

Ground tissue

Only element of the ground tissue are isodiametrical (more or less spherical) parenchyma cells. Rays are missing.

Microscopic cross-section of a fiber bundle of the central trunk area embedded in parenchymatous ground tissue (MX = Metaxylem, F = Fiber, PA = parenchyma)

Vascular bundles

Vascular bundles consist of xylem, phloem, axial parenchyma (ground tissue) and fibrous tissue. There are also reduced vascular bundles, consisting almost exclusively of fibres. In Euterpe edulis these can be found in high numbers at the peripheral zone of the stems of older plants. A vascular bundle contains one or more large vessels (metaxylem), which are responsible for the water transport and surrounded by a parenchymatous tissue. E. edulis has vascular bundles with predominantly one but also some with two metaxylems. Arranged towards the bark is the phloem, consisting of a line of thin-walled sieve cells and parenchyma. The Phloem, which serves for the nutrient transport, is surrounded by radial wide-stretched fibre sheath (protective sheath consisting out of sclerenchymatous fibres), which determines considerably the wood density. Peripheral vascular bundles have extremely thick fibres with a small lumen. Outwards, the fibres and lumen are getting larger while wall thickness is decreasing. In contrast to the fibre cells of the well known dicotyledonous timbers fibre cells of the palms are more durable. The thickening of the cell walls occurs continuously.

Part of the microscopic cross-section of a vascular bundle (MT = Metaxylem, F = Fibre, PL = phloem)

Microscopic cross-section of fibres. Left: fibre cells becoming thin-walled from Metaxylem outward. Right: thick-walled fibre cells with extremely small lumina enclosing the phloem

Microscopic cross-section of fiber- (F) and thin-walled parenchyma cells (PA)

Highly magnified microscopic cross section of fiber- (F) and parenchyma cells (PA)

Longitudinal section of the isodiametric (more or less spherical) cells of the parenchymatous ground tissue

Helical wall thickening of vessel elements from the Protoxylem

Highly magnified microscopic longitudinal section of the vessel elements of the protoxylem