d.

e.

power. Note the larger rectangular cells, belonging to the deeper layers, with well-marked currents in them, which carry the chlorophyll granules round and round inside the cell-wall.

If no currents are seen at first, gently warm the leaf by immersing it for a short time in water heated to a temperature between 30° and 35° C.

Anacharis. Take a yellowish-looking leaf: mount in water and examine with a high power; the phenomena observed are like those in Vallisneria. They are best observed in the single layer of cells at the margin of the leaf.

Nettle-hair. Mount an uninjured hair in water with the bit of leaf to which it is attached (it is essential that the terminal recurved part of the large cell forming the hair be not broken off); examine with the highest available power: currents carrying along very fine granules will be seen in the cell, their general direction being that of its long axis.

VII.

THE BRACKEN FERN (Pteris aquilina).

THE conspicuous parts of this plant are the large green leaves, or fronds, which rise above the ground, sometimes to the height of five or six feet, and consist of a stem-like axis or rachis, from which transversely disposed offshoots proceed, these ultimately subdividing into flattened leaflets, the pinnules. The rachis of each frond may be followed for some distance into the ground. Its imbedded portion acquires a brown colour, and eventually passes into an irregularly branched body, also of a dark-brown colour, which is commonly called the root of the fern, but is, in reality, a creeping underground stem, or rhizome. From the surface. of this, numerous filamentous true roots are given off. Traced in one direction from the attachment of the frond, the rhizome exhibits the withered bases of fronds, developed in former years, which have died down; while, in the opposite direction, it ends, sooner or later, by a rounded extremity beset with numerous fine hairs, which is the apex, or growing extremity, of the stem. Between the free end and the fully formed frond one or more processes, the rudiments of fronds, which will attain their full development in following years, are usually found.

The attachments of the fronds are nodes, the spaces between two such successive attachments, internodes. It

A

will be observed that the internodes do not become crowded towards the free end, and there is nothing comparable to the terminal bud of Chara with its numerous rudimentary appendages.

When the fronds have attained their full size, the edges of the pinnules will be observed to be turned in towards the underside, and to be fringed with numerous hair-like processes which roof over the groove, enclosed by the incurved edge. At the bottom of the groove, brown granular bodies are aggregated, so as to form a streak along each side of the pinnule. The granules are the sporangia, and the streaks formed by their aggregation, the sori.

Examined with a magnifying glass, each sporangium is seen to be pouch-shaped, like two watch-glasses united by a thick rim. When ripe, it has a brown colour, readily bursts, and gives exit to a number of minute bodies which are the spores.

The plant now described is made up of a multitude of cells, having the same morphological value as those of Chara, and each consisting of a protoplasmic mass, a nucleus and a cellulose wall. These cells, however, become very much modified in form and structure in different regions of the body of the plant, and give rise to groups of structures called tissues, in each of which the cells have undergone special modifications. These tissues are, to a certain extent, recognizable by the naked eye. Thus, a transverse section of the rhizome shews a circumferential zone of the same dark-brown colour as the external epidermis, enclosing a white ground-substance, interrupted by variously disposed bands, patches, and dots, some of which are of the same dark-brown hue as the external zone, while others are of a pale yellowish-brown.

The dark-brown dots are scattered irregularly, but the major part of the dark-brown colour is gathered into two narrow bands, which lie midway between the centre and the circumference. Sometimes the ends of these bands are united. Enclosed between these narrow, dark-brown bands are, usually, two elongated, oval, yellowish-brown bands; and, outside them, lie a number of similarly coloured patches, one of which is usually considerably longer than the others.

A longitudinal section shews that each of these patches of colour answers to the transverse section of a band of similar substance, which extends throughout the whole. length of the stem; sometimes remaining distinct, sometimes giving off branches which run into adjacent bands, and sometimes uniting altogether with them.

At a short distance below the apex of the stem, however, the colour of all the bands fades away, and they are traceable into mere streaks, which finally disappear altogether in the semi-transparent gelatinous substance which forms the growing end of the stem. Submitted to microscopic examination, the white ground-substance, or parenchyma, is seen to consist of large polygonal cells, containing numerous starch granules; and the circumferential zone is formed of somewhat elongated cells, the thick walls of which have acquired a dark-brown colour, and contain little or no starch. The dark-brown bands, on the other hand, consist of cells which are so much elongated as almost to deserve the name of fibres and constitute what is termed sclerenchyma. Their walls are very thick, and of a deep-brown colour; but the thickening has taken place unequally, so as to leave short, obliquely directed, thin places, which look like clefts. The yellow bands, lastly, are vascular bundles. Each consists, externally, of thick

walled, elongated, parallel-sided cells, internal to which lie elongated tubes devoid of protoplasm, and frequently containing air. In the majority of these tubes, and in all the widest, the walls are greatly thickened, the thickening having taken place along equidistant transverse lines. The tubes have become flattened against one another, by mutual pressure, so that they are five- or six-sided; and, as the markings of their flattened walls simulate the rounds of a ladder, they have been termed scalariform ducts or vessels. The cavities of these scalariform ducts are divided at intervals, in correspondence with the lengths of the cells of which they are made up, by oblique, often perforated, partitions. Among the smaller vessels, a few will be found, in which the thickening forms a closely wound spiral. These are spiral vessels.

The rachis of a frond, so far as it projects above the surface of the ground, is of a bright green colour; and, in transverse section, it presents a green ground-substance, interrupted by irregular paler markings, which are the transverse sections of longitudinal bands of a similar colour. There are no brown spots or bands. Examined microscopically, the ground-substance is found to be composed of polygonal cells containing chlorophyll. These are invested superficially by an epidermis, composed of elongated cells, with walls thickened in such a manner as to leave thin circular spots here and there. Hence, those walls of the cells, which are at right angles to the axis of vision, appear dotted with clear spots; while, in those walls of which transverse sections are visible, the dots are seen to be funnel-shaped depressions.

The pale bands are vascular bundles containing scalariform and spiral vessels. The outer layer investing each is chiefly formed of long hollow fibres with very thick