B.

5.

Within the sheath-cells a layer of small parenchymatous cells containing starch.

Within the last layer come the bast-cells; small rectangular cells with slightly thickened walls, and arranged in several rows.

Within the bast layer, come one or more rows of larger thin-walled cells.

The cross sections of the vessels: their greatly thickened walls, and large central cavity containing no protoplasm.

Scattered here and there, in the spaces between the angles of the vessels, are small parenchymatous cells containing starch-granules.

Treat with iodine: the protoplasm stained brown; the starch-granules deep blue, rendering some of the cells quite opaque and almost black-looking,

Cut a thin longitudinal section of the stem and examine with 1 inch and then with obj. Make out the various tissues described in 3 and 4.

α. The epidermis and parenchyma, much as in the transverse section.

b.

C.

a.

The sclerenchyma is seen to be made up of greatly elongated cells, tapering towards each end.

The vascular bundles; note in them

The sheath-cells and parenchyma much as in the transverse section.

B. The vessels: elongated tubes presenting cross partitions at long intervals. Two forms of vessel will be seen, viz. scalariform vessels,

6.

1.

with regular transverse thickenings on their walls and spiral vessels, less numerous than the last form with a continuous spiral thickening on their walls.

The bast-cells: seven or eight times as long as they are broad, and terminating obliquely at each end.

The elongated larger cells (4. d. 8.): they have very slightly thickened walls and no scalariform markings.

[Cut off half an inch of the growing end of the stem, imbed it in paraffin upside down, and cut a series of transverse sections: examine them with the microscope, beginning with those farthest from the growing point. At first the various tissues described in 3 and 4 will be readily recognisable; as the sections nearer the growing point are examined they will be less distinct, and close to the growing point the whole section will be found to be composed entirely of parenchymatous closely fitting cells.]

[c. The leaf. Imbed a leaf in paraffin and cut a thin vertical section: examine with 1 inch obj. It will be found to be constructed essentially on the same plan as the leaf of the bean. (VIII.)]

d. The reproductive organs.

Examine a sorus with 1 inch obj. without a cover-glass. It is composed of a great number of minute oval bodies, the sporangia.

2. Scrape off some sporangia and mount in water: examine with 1 inch obj.

a.

Their form: they are oval biconvex bodies borne on a short stalk.

b. Their structure: composed of brownish cells, one row of which has very thick walls, and forms a marked ring (annulus) round the edge of the sporange.

C.

Their mode of dehiscence (look out for one that has opened): by a cleft running towards the centre of the sporange from a point where the annulus has torn across.

3. Burst open some sporangia by pressing on the coverglass: examine with obj. the spores which are set free. a. Their size: measure.

b.

Their form: somewhat triangular.

[c. Their structure; a thick outer coat, a thin inner coat, protoplasm, and a nucleus: crush some by pressure on the cover-glass.]

B. THE PROTHALLUS; SEXUAL GENERATION.

Prothalli may be obtained by sowing some spores on a glass slide, and keeping them warm and very moist for about three months. They are small deep green leaf-like bodies.

1.

a. The Prothallus.

Transfer a prothallus to a slide, and mount it in water with its under surface uppermost. Examine with 1 inch obj.

α.

Its form: a thin kidney-shaped expansion from which, especially towards its convex border, a number of slender filaments (rootlets) arise.

b. Its structure.

α. The leafy expansion: it consists throughout most of its extent of a single layer of polyhe

C.

dral chlorophyll-containing cells, but at parts it is more than one cell thick.

B. The rootlets: composed of a series of cells which contain no chlorophyll.

The antheridia and archegonia: these can just be seen with an inch objective as minute eminences on the under surface of those parts of the prothallus which consist of several layers of cells.

b. The reproductive organs.

These are to be found by examining the under surface of the prothallus with obj.

1.

The antheridia. Most numerous near and among the
rootlets.

a. Their form: small hemispherical eminences.
b. Their structure: made up of an outer layer
of cells containing a few chlorophyll-granules
and through which can be seen a number of
smaller cells which occupy the centre of the organ:
in the latter cells, in ripe antheridia, spirally
coiled bodies (antherozooids) can be indistinctly

seen.

2. The antherozooids.

Some of these are sure to be found swimming about in the water if a number of ripe prothalli are examined. a. Small bodies, coiled like a corkscrew, thick at

b.

one end, and tapering towards the other, which has a number of cilia attached to it. To the thicker end of the antherozooid is often attached a rounded mass containing colourless granules. Treat with iodine; this stains them and stops

3.

their movements so that their form can be more distinctly seen.

The archegonia. Most numerous towards the concave border of the prothallus.

a. Their form: chimney-shaped eminences with a small aperture at the apex.

b.

Their structure: composed of a layer of transparent cells, containing no chlorophyll, arranged in four rows, and surrounding a central cavity which extends into the cushion formed by the thickened part of the prothallus (a. 1. b. a). In this cavity lies, in young specimens, a large nucleated granular basal cell, with two or three smaller granular cells above it in the narrow upper part of the cavity; in older specimens this upper part is empty, forming a canal leading down to the basal cell.

4. Examine young Fern in connexion with its prothallus.