In a paper (Russian) recently read before the Botanical Section of the
St. Petersburg Natural History Society, Mr. K. Friderich describes in
detail the anatomical structures to be met with in the aerial roots of
_Acanthorhiza aculeata_, these roots presenting a remarkable example of
roots being metamorphosed into spines. Supplementing this, E. Regel made
the following remarks:
Palm trees, grown from seed, thicken their stems for a succession of
years, like bulbs, only at the base. Many palms continue this primary
growth (i.e., the growth they first started with) for fifty to sixty
years before they form their trunk. During this time new roots are
always being developed at the base of the stem, in whorls, and these
always above the old roots. This even takes place in old specimens,
especially in those planted in the open ground which have already formed
a trunk, In such cases the cortex layer, where the roots break through,
is sprung off. In conservatories, under the influence of the damp air,
this root formation, on which indeed the further normal growth of the
palm depends, takes place without any special assistance. When the palm
is grown in a sitting room, one must surround the base of the trunk with
moss, which is to be kept damp, in order to favor the development of the
roots. When the base of the palm trunk has almost reached its normal
thickness, then begins the upward development of the trunk, which takes
place more slowly in those species whose leaves grow close together than
in those whose leaves are further apart. In specimens of many species of
Cocos and Syagrus, whose leaves are particularly far apart, the stems
grow so quickly when planted in the open ground that they increase by
five to six feet in height per annum. The stem of those palms which
develop a terminal inflorescence have ended their apical growth by doing
so, and wither gradually, In addition to this (withering) in the case,
e.g. of _Arenga saccharifera_, new inflorescences are developed from the
original axils _(Blattachseln)_ from above downward, so that one sees at
last the already leafless trunk still developing inflorescences in the
direction toward the base of the trunk. Almost all palms with this
latter kind of growth develop offshoots in their youth at the base of
their trunks, which shoot up again into trunks after the death of the
primary trunk, if they are not taken off before. As to the structure of
the palm trunks out of unconnected wood bundles, the assertion has been
made that the palm stem does not grow thicker in the course of time, and
that this is the explanation of the columnar almost evenly thick trunk.
But careful measurements that were made for years have led Regel to the
conclusion that a thickening of the trunk actually takes place, which
probably amounts to an increase of about a third over the original
circumference of the trunk.
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