In some Lepidodendron stems a second triangular depression may occur above the ligular pit, the meaning of which is not clear: this has been called the triangulum by Potonié[250]. Stur[251] suggested that it may represent the position occupied by a sporangium in Lepidodendron cones.
It is important to remember that as a branch increases in girth the leaf-cushions are capable of only a certain amount of growth: when the limit is reached they are stretched farther apart and thus the narrow groove which separates them is converted in older stems into a comparatively broad and flat channel, thus altering the surface characters.
Fig. 146. Lepidophloios and Lepidodendron leaf-cushions.
- A, B, D, F, G, H, I. Lepidophloios. (Fig. A should be reversed.)
- C, E. Lepidodendron aculeatum.
- A, B. From a specimen in the Sedgwick Museum, Cambridge (leaf-cushion 3 cm. broad).
- C. From a specimen in the Sedgwick Museum, Cambridge (leaf-cushion 4 cm. long).
- D. From a section in the Cambridge Botany School Collection.
- E. From a specimen in the Bunbury Collection, Cambridge Botany School, showing Spirorbis shells (leaf-cushion 2 cm. long).
- F. From a section in the Williamson Collection, British Museum No. 1, 973.
- G, H, I. From sections in the Cambridge Botany School Collection.
Another feature worthy of notice in reference to the leaf-cushions of Lepidodendron is the occurrence in rare instances of alternate zones of larger and smaller cushions. This variation in the size of the leaf-cushions is by no means uncommon in the closely allied genus Sigillaria; in Lepidodendron it has been described by Potonié[252] in L. volkmannianum and more recently by Mr Leslie and myself[253] in a South African species L. vereenigense.
Owing to the natural exfoliation of the superficial layers of the outer bark at a certain stage in the growth of the plant, or in some instances no doubt as the result of post-mortem decay, which destroys the delicate cells of the meristematic zone in the outer cortex, isolated leaf-cushions and strips of the external surface are occasionally met with as carbonised impressions.
The appearance presented by a Lepidodendron stem which has been deprived of its superficial tissues may be dealt with more intelligibly after we have become familiar with the anatomical characters.
iii. Lepidophloios.
Before proceeding further with the genus Lepidodendron a short account may be intercalated of the external features of a lepidodendroid type of stem which it is customary to describe under a distinct generic title Lepidophloios. This name is convenient for diagnostic purposes though it seems clear that apart from the form of the leaf-cushion ([fig. 146], A) we are at present unable to recognise any well-defined differences between the two forms Lepidodendron and Lepidophloios. For general purposes the name Lepidodendron will be used as including plants possessing leaf-cushions of the type already described as well as those with the Lepidophloios form of cushion.