The above observations were made with a battery of five Grove cells; the reduction of cells from 5 to 2 made no difference in the phenomena except in reducing their brilliancy.

Reasoning from the above observations, it seems evident that the effect of the higher tension is to break up a compound, or compounds, of which C, D, and 447 represent constituent elements; while, at the same time, it would appear that 667 represents a line of some compound which is simultaneously dissociated.

The unequal behaviour of the lines has been further noted in another experiment, in which the products of distillation of bröggerite were observed in a vacuum tube and photographed at various stages. After the first heating, D, and 4471 were seen bright, before any lines other than those of carbon and hydrogen made their appearance. With continued heating, 667, 5016, and 492 also appeared, although there was no notable increase of brightness in the yellow line; still further heating introduced additional lines 5048 and 6347.

These changes are represented graphically in the following diagram (fig. 2).

FIG. 2.-Diagram showing order in which lines appear in spectrum of vacuum tube when bröggerite is heated.

It was recorded further that the yellow line was at times dimmed, while the other lines were brightened.

In my second note, communicated to the Royal Society on the 8th instant, I stated that I had never once seen the lines recorded by Thalén in the blue, at X 4922 and 4715.

It now seems possible that their absence from my previous tubes was due to the fact that the heating of the minerals was not sufficiently prolonged to bring out the gases producing these lines.

It is perhaps to the similar high complexity of the gas obtained from clèveite that the curious behaviour of a tube which Professor Ramsay was so good as to send me, must be ascribed. When I received it from him, the glorious yellow effulgence of the capillary, while the current was passing, was a sight to see. But after this had gone on for some time, while the coincidence of the yellow line with D of the chromosphere was being inquired into, the luminosity of the tube was considerably reduced, and the colours in the capillary

and near the poles were changed. From the capillary there was but a feeble glimmer, not of an orange tint, while the orange tint was now observed near the poles, the poles themselves being obscured by a coating on the glass of brilliant metallic lustre.

After attempting in vain for some time to determine the cause of the inversion of D, and 447 in various photographs I had obtained of the spectra of the products of distillation of many minerals, it struck me that these results might be associated with the phenomena exhibited by the tube, and that one explanation would be rendered more probable if it could be shown that the change in the illumination of the tube was due to the formation of platinum compounds, platinum poles being used. On May 21st I accordingly passed the current and heated one of the poles, rapidly changing its direction to assure the action of the negative pole, when the capillary shortly gave a very strong spectrum of hydrogen, both lines and structure. A gentle heat was continued for some time and apparently the pressure in the tube varied very considerably, for as it cooled the hydrogen disappeared and the D3 line shone out with its pristine brilliancy. The experiment was repeated on May 24th and similar phenomena were observed.

III. Further Observations on the Organisation of the Fossil Plants of the Coal-measures. Part III. Lyginodendron and Heterangium." By W. C. WILLIAMSON, LL.D., F.R.S., Emeritus Professor of Botany in the Owens College, Manchester, and D. H. SCOTT, M.A., Ph.D., F.R.S., Honorary Keeper of the Jodrell Laboratory, Royal Gardens, Kew. Received May 14, 1895.

(Abstract.)

Introduction.

The two genera, Lyginodendron and Heterangium, are among the most interesting and at the same time the most puzzling representatives of the Carboniferous flora. Although we are still without any satisfactory evidence as to the reproductive organs in either genus, yet the organisation of their vegetative members is preserved with such completeness and perfection as to show that these fossils present a combination of characters such as exists in no living group of plants.

The evidence afforded by the vegetative characters clearly points to a position intermediate between ferns and Cycades.

I. LYGINODENdron.

Lyginodendron oldhamium, Will.,* is one of the commonest fossils preserved in the calcareous nodules of the Lancashire and Yorkshire coal-measures, and has also been found in those of Germany and Austria. A renewed investigation, with the aid of numerous additional specimens, has enabled us to clear up many doubtful points in the structure of the plant, and to give for the first time a complete account of all its vegetative organs.

A. The Stem.

1. General Structure.--The middle of the central cylinder or stele is occupied by a parenchymatous pith. Surrounding this is the primary wood, which usually forms a ring of from five to eight distinct strands. Beyond this we find, in all but the youngest specimens, a broad zone of secondary wood, then the cambium, and next the phloëm. The whole stele is bounded by a well-marked pericycle. The inner cortex is mainly parenchymatous, while the outer zone consists of alternating strands of fibres and parenchyma, constituting the wellknown "dictyoxylon cortex " of Count Solms-Laubach.

The pericycle and cortex are traversed by the leaf-trace bundles, which alternate with the perimedullary xylem-strands.

2. Course of the Vascular Bundles.—We have obtained direct proof that the perimedullary strands of xylem form the downward continuation of the bundles which pass out into the leaves. Thus the entire bundle-system of the stem is built up of the leaf-traces. Each leaftrace extends through at least ten internodes; five internodes are traversed while it is passing through cortex and pericycle, and five more after it has reached the periphery of the pith. On entering the pith the trace turns aside in the kathodic direction, and unites with the adjacent perimedullary strand on that side. We thus see that these strands are sympodial bundles, made up of the united lower portions of adjacent leaf-traces.

In the upper part of its course, each leaf-trace consists of two bundles, which unite into one in passing through the pericycle.

The phyllotaxis was usually two-fifths, but in the smallest stems was probably one-third.

3. Structure of the Vascular Bundles.-The preservation is so good that we have been able to determine with certainty that the bundles in the stem were normally collateral, having xylem on their inner, and phloëm on their outer side. As they passed out into the leaves

* See Williamson, "Organisation of the Fossil Plants of the Coal-measures, Part IV," Phil. Trans.,' 1873; Part VI, Phil. Trans.,' 1874; Part VII, ‘Phil. Trans.,' 1876; Part XIII, 'Phil. Trans.,' 1887; Part XVII, 'Phil. Trans.,' 1890.

their structure became concentric, the phloëm here extending all round the xylem.

The xylem of the bundles in the stem of Lyginodendron exactly resembles that in the leaves of existing Cycades. The protoxylem lies in the interior of the primary wood, but near its outer side, So that the greater part of the primary wood was centripetally developed, while a smaller portion was centrifugal. We propose to term such bundles mesoxylic or mesarch.* All statements as to the position of the protoxylem are based on longitudinal as well as on transverse sections.

4. The Secondary Tissues.-A few young stems have been observed with little or no secondary thickening; in most specimens it has made considerable progress. A large amount of secondary wood and bast, both fascicular and interfascicular, was formed, by means of a normal cambial layer, which is often well preserved.

The tracheides of the wood have numerous bordered pits on their radial surfaces. Similar elements occur in the primary wood also. The rows of tracheides are separated by numerous medullary rays.

The phloem is often well preserved, so that primary and secondary phloëm can be distinguished.

The secondary tissues bear a general resemblance to those in the stems of Cycades.

5. Pith and Pericycle.-Both these tissues contained nests of dark coloured elements, probably of a sclerotic nature. They are also traversed by numerous rows of cells with carbonaceous contents, which may have been secretory sacs, but not intercellular canals. At the outer border of the pericycle a characteristic internal periderm was developed.

6. The Cortex. The parenchymatous portions of the outer cortex became much dilated in the older stems, in consequence of the secondary growth.

7. On Small Stems of the Lyginodendron Type.-Certain very small stems have been described, differing in structure from the usual form. In some of these the primary xylem forms a continuous ring, instead of being divided into distinct bundles. We now suggest that these specimens may represent the basal, first-developed region, of normal stems. In Osmunda, which in many respects resembles Lyginodendron, it has been shown by M. Leclerc du Sablon, that the embryonic stem has the same peculiarity.

8. Structural Anomalies.-Some of the specimens show remarkable individual anomalies, the most frequent and conspicuous of which consists in the appearance of a cambium at the periphery of the pith,

* One of the authors has recently found that this peculiarity sometimes extends to stem-structures in Cycades; in the peduncles of both male and female flowers of Stangeria the bundles are often mesoxylic.

forming medullary wood and bast, with inverted orientation. This is precisely the anomaly shown by certain species of Tecoma, and other dicotyledons. The anomalous medullary cambium is continuous with the normal cambium through the leaf-trace gaps. This case is a striking instance of the independent appearance of the same structural peculiarity in families as remote as possible from each other.

B. The Leaf.

1. Connexion between Leaf and Stem.-New and conclusive evidence has been found, confirming the conclusion previously arrived at (in Mem. XVII), that "Rachiopteris aspera" is the petiole of Lyginodendron. In several specimens petioles with the characteristic structure of that fossil, are found inserted on the stems of Lyginodendron. The vascular bundles on leaving the pericycle of the stem bend out rapidly into the base of the leaf, becoming concentric at the same time. Petioles, continuous with the Lyginodendron stem, have been traced up to the point where they begin to ramify.

2. Form of the Leaf.—The petioles, which we now know to belong to our plant, branch repeatedly, and ultimately give rise to small palmately-segmented leaflets. The leaf was thus a highly compound one, and we can confirm the statement previously made, that the character of the foliage was that of Brongniart's form-genus Sphenopteris.

3. Structure of the Petiole.-The most important point here is that throughout the petiole and rachis, the vascular bundles, of which either one or two are present, are of typical concentric structure, as in a fern.

The cortex of the petiole has essentially the same structure as that of the stem.

4. Structure of the Lamina.-We have examined sections of leaflets (found in connexion with petioles of Lyginodendron), in which the structure is perfectly preserved. The lamina had a distinctly bifacial structure, with well-differentiated palisade, and spongy parenchyma. Stomata have only been observed on the lower surface. The vascular bundles in the lamina appear to have been collateral, as is also the case in recent ferns.

5. On a Peculiar Bud-like Structure.-This is a unique specimen, consisting of an axis, of obscure structure, bearing numerous appendages which exactly resemble the well-known cortical outgrowths of the stem and petiole of Lyginodendron. At first sight, the specimen bears some resemblance to a cone, but it was more probably a bud or young leaf, from which the inner delicate tissues have perished, leaving the protective outer coat, bearing the appendages, which may represent the bases of paleæ.