(9) Rapid and irregular currents are unfavourable for extending distribution. This is well seen in the case of the comparatively unproductive almost perpendicular sides of the Bass Rock and the western side of the May Island, as well as in many other parts of the coast of the mainland, which are at all times exposed to the full force of the waves from deep water. The result is the removal of algæ from their substratum by the destruction of their rhizoids, or the destruction of the thalli, and the prevention of the maturation of fruit. Stunted growth is almost invariably found in such places. Specimens of Porphyra vulgaris, growing in an exposed situation on the Bass Rock, were examined with respect to the maturation and emission of spores at a time when other species on sheltered spots on the island of Fidra were found to be ripe. No signs of maturity were found, however, although the cells of the stunted thallus were intact, and had the normal appearance of those in a healthy plant. Even in rocky areas, the bare exposed places carry fewer plants belonging to fewer genera than do the more sheltered crevices or crannies.

(10) The bright colour of some sea-weeds seems to attract a number of animals, which aid in bringing about cross fertilisation, e.g., marine Infusoria. Moreover, the movements of minute Crustacea and other small animals tend in the same direction.

(11) The effects of light are twofold--(a) it influences the swarmspores immediately subsequent to their emission; and (b) it affects the physiological activities of the mature plants. No observations have as yet been made on the penetrability of luminous rays into the extensive waters of the Forth, but from the large amount of suspended matter in the water, which, moreover, varies very greatly in amount at different times, it cannot be doubted that the rays that reach those algae growing in the deeper parts also vary correspondingly in intensity, while all the dredgings that have been made go to show that it is those alga whose colour is of the brightest red that occur at the greatest depths. These are succeeded by sherry-coloured species and by Phæophyceæ, in shallower, and therefore, on the whole, more luminous parts; while they, in turn, give

place to Chlorophyceae, where the light is most intense, and most nearly simulates the conditions of ordinary land or clear fresh water vegetation.

On the general question of depth Meneghini speaks as follows" A hundred feet beneath the surface of the sea, living algæ are rarely met with, but some are attached to masses detached by accident from higher rocks, and these speedily perish. The number of species decreases on descending into the depths of the sea. . Beyond forty feet from the surface, Ulvæ are rarely met with; beyond sixty, there are no Ceramiæ; beyond a hundred, no longer Fucoidea, and the vegetable kingdom ceases, leaving the field free for animals, which are always distributed in those abysses with great richness of form and tint."

Kjellman, whilst asserting that it is at present impossible to state the causes of a difference of facies of marine vegetation at different depths, until further advances have. been made in vegetable physiology and hydrography, goes on to say "Bis auf Weiteres müssen wir annehmen, dass an der Küste von Bohuslän noch bei einer Tiefe von 1 bis 2 Faden die Temperatur des Meereswassers, sein Gehalt an fixen Bestandtheilen und an Gasen, der Wasserdruck, die Menge und Beschaffenheit des bis in diese Tiefe hinabdringenden Lichtes u.s.w. für die meisten der Chlorophyllophyceen und Nostocaceen vortheilhaft sind, dass aber erst in grössere Tiefe diese Verhältnisse so modificirt werden dass sie den meisten Rhodospermen-Arten angemessen und gunstig sind."

It may further be observed generally-

(1) That many species occur in localities where the external environments are subject to very much greater changes than in other areas. There is, indeed, a regular gradation in this respect (a) from the upper limits of the tide to the bottom of the deeper parts of the estuary, and (b) these changes become more pronounced as the water shallows in the more landward parts.

(2) Some marine algæ have a predilection for conditions unsuitable for almost all others, e.g., Enteromorpha intestinalis, which occurs freely in pools above high-water mark, which acquire their saline constituents from spray only.

(3) A comparison of the various areas referred to above. shows that they are by no means all of equal value with respect to number of different species, some having a rich and diverse floor, others being more uniform and poor. Generally, with certain limitations, uniformity increases with depth.

(4) The general facies of any given area, as of the entire estuary, varies in summer as compared with winterthe disappearance of many species during winter producing marked changes.

(5) Transition from one bathymetrical area to another takes place gradually, and not by any sudden or sharp lines of demarcation; species characteristic of one being almost invariably found in other higher and lower belts, and the same is true with respect to changes observed in horizontal extension.

On Certain Points in the Morphology of Frullania and some other leafy Jungermannieæ. By Professor ALEXANDER DICKSON, M.D. (Plate XV.)

(Part of Presidential Address, delivered 10th December 1885.)

The subject I have chosen for a few special remarks, on this occasion, is one connected with the morphology of certain of the Foliose Jungermannieæ or Scale-Mosses.

In these plants Leitgeb describes the development of the shoot as taking place by the cutting off of successive segments from an apical cell. This apical cell is in the form of an inverted three-sided pyramid, and from this pyramid segments are successively cut off by septa parallel to its sides. Each cell-segment thus cut off becomes divided by a septum parallel to its free surface into two cells, of which the superficial one-which may be called the Initial Leafcell goes to form the leaf, while the deeper one goes to form the corresponding portion of the axis.* Here we seem to have the most complete realisation of the idea of the leafy plant as built up of successive members-phyta or plant-individuals-each consisting of a leaf with a

*

Leitgeb, Untersuch, ü. d. Lebermoose, Heft ii. p. 5.

corresponding portion of the axis-a doctrine to which I have always been disposed to adhere, however transcendental it may seem to some minds.

In the more typical forms, the shoot exhibits a markedly bilateral character, with two surfaces—a dorsal away from, and a ventral towards the surface along which the plant lies. The leaves are alternate, and are arranged in three longitudinal series (corresponding to the successive segments cut off from the three sides of the apical cell), of which two are dorsi-lateral and the third ventral. The leaves of the two dorsi-lateral series resemble each other in all respects, their insertion is more or less oblique, and they are termed Lateral Leaves (Pl. XV. U). The leaves of the ventral series are of a different configuration, their insertion is more or less transverse, and they are termed Amphigastria, or Amphigastrial Leaves (Pl. XV. am).

In a great number of these bilateral forms, the lateral leaves exhibit a more or less marked division into two lobes, which, it would appear, originate from the two cells resulting from the first division of the Initial Leaf-cell. In some cases-e.g., many species of Jungermannia-the lobes are equal and similar; but in others the upper lobe is comparatively large, while the lower one is smaller. The lower lobe is usually called the Auricle. This Auricle may be flat, as in Madotheca (Porella), or remarkably pouched, so as to form the curious helmet-shaped body seen in Frullania.

In connection with the development of the lateral branches of the Foliose Jungermannieæ, Leitgeb has made certain observations which involve questions of the greatest importance as to the relation between axis and leaf; and to these I would now direct your attention. To put the matter shortly, Leitgeb asserts that in a considerable number of these plants the lateral branch is developed from the lower of the two cells into which the Initial Leaf-cell subdivides; and that, in consequence, the lateral branch appears as replacing, and therefore being potentially equivalent to, the lower lobe of the lateral leaf: e.g., in Frullania, Madotheca, Lepidozia, &c. In other cases, as in that of Radula, he asserts that only the lower portion of the lower half of the Initial Leaf-cell goes to form the lateral branch, leaving the other portion to form an auricle.

With regard to Frullania and Madotheca (Porella), Leitgeb gives the following explanation of his views:

"In Frullania," he says, "if the leaf from whose base a branch springs be carefully examined, one is readily convinced that it wants the helmet-shaped lower lobe (the auricle), and sees that the branch arises exactly in the place of this lobe. On its ventral aspect the branch is subtended by a leaf directed obliquely towards the base of the main axis; and from its position, certainly, this leaf might be viewed as belonging to the main axis, and as representing the lower leaf-lobe, did not its form-corresponding exactly with that of the amphigastria—and the complete absence of any connection with the corresponding upper leaf-lobe make such an assumption seem questionable. The history of its development, moreover, shows that this leaf belongs to the lateral branch, being, in fact, its first leaf, of the amphigastrial series. I have given this example first because here the peculiar helmet-like form of the lower lobes prevents their being confounded with the very differently shaped amphigastria. In Madotheca the relations are not so manifest, since here the lower leaf-lobes pretty closely resemble the amphigastria; but here, also, a more careful examination shows that the leaflet obliquely subtending, on the ventral aspect, the place of origin of the branch is not a lower leaf-lobe belonging to the main axis, but is the first amphigastrial leaf of the lateral branch."*

In support of this view, Leitgeb appeals to the development of the parts from the segments successively cut off from the apical cell, and states that the lower half of the Initial Leaf-cell, instead of going to form a lower leaflobe, or auricle, assumes the character of an apical cell, and goes on to the development of a lateral branch.

If these lateral branches originate in substitution for leaf-auricles, as is stated by Leitgeb, it cannot but be regarded as very surprising, and as subversive of our ordinary ideas of the relation of leaf to axis. A philosopher, like Mr Herbert Spencer,† may propound the morphological absurdity of the substitution of flower-buds for the petals and stamens of the flower of an Umbellifer, without its calling for serious comment; but when a

Leitgeb, "Ueber die Verzweigung der Lebermoose," Bot. Zeit., 1871, pp. 557 et seq.

+ Principles of Biology, vol. ii., Appendix A.