Stur[303] held the view that the depressions on Ulodendron stems represent the places of attachment of special shoots comparable with the bulbils of Lycopodium Selago, or, it may be added, with the short branches occasionally produced on Cycas stems. If the depressions were formed by the pressure of the bases of cones, it is clear that the size of the cavity must be an index of the diameter of the cone. The larger Ulodendron scars exceed in diameter the base of any known lepidodendroid strobilus. Another obvious difficulty, which has not been overlooked by Kidston who holds that the scars were produced by sessile cones, is that in Lepidodendron Veltheimianum strobili were borne at the tips of slender branches; the same difficulty is presented by Bothrodendron (Fig. 213). It is unlikely that two types of strobili were produced on the same plant, particularly as the cone of L. Veltheimianum was heterosporous.
The cones of certain species of Pinus remain attached to the tree for many years and their bases become embedded in the stem; this is particularly well shown in the drawing of a cone of Pinus clausa ([fig. 159]), for which I am indebted to Mr Sudworth, Dendrologist in the United States Forest Service. Mr Sudworth has drawn my attention to P. attenuata and P. muricata in illustration of the same phenomenon[304]. The example shown in [fig. 159] cannot, however, be matched by any known specimen of Ulodendron; in the case of the depressions on the stem of a Pine the cone-base fits the circular scar, but in the fossil stems it is practically certain that this was not the case.
Fig. 159. Pinus clausa. ½ nat. size.
There can be little doubt that certain Palaeozoic Lycopods shed their branches by a method similar to that employed by the Kauri Pine of New Zealand and by some species of Dicotyledons. The evidence adduced in the case of Bothrodendron punctatum is a strong argument in favour of extending the same explanation to other Ulodendron shoots.
Fig. 160.
- Lepidophloios scoticus Kidst. From a specimen from the Calciferous Sandstone, Midlothian, in Dr Kidston’s Collection; rather less than ⅓ nat. size.
- L. scoticus cone. From a specimen from the Calciferous Sandstone of Midlothian in Dr Kidston’s Collection; slightly reduced.
b. Halonia.
The branched axis with Lepidophloios leaf-cushions, represented in [fig. 160], A, illustrates a special form of shoot described by Lindley and Hutton[305] under the generic name Halonia. The original specimens referred to this genus are decorticated axes showing remains of Lepidodendroid leaf-cushions. The spirally disposed circular scars in the specimen of Halonia (Lepidophloios scoticus[306]) shown in [fig. 160] constitute the characteristic feature of the genus; they may have the form, as in [fig. 160], A, of circular discs with a central umbilicus marking the position of a vascular strand, or, as in the sandstone cast of Halonia tortuosa shown in [fig. 161][307], they may appear as prominent tubercles. The latter example illustrates the condition characteristic of partially decorticated stems.