the Phanerogams there is usually a single embryo-sac mother-cell which divides into four daughter-cells; three of these do not develop further, while the fourth becomes the embryo-sac. The formation of the PROTHALLIUM (or ENDOSPERM) and of the ARCHEGONIA (or EGG-CELLS) differs in the several classes of Phanerogams. The fertilised ovum

grows into the embryo while still enclosed within the macrospore and at the expense of the parent plant. When the embryo has reached a certain stage in its development, which is different and characteristic in different plants, its growth is arrested, and after the separation from the parent plant it undergoes a period of rest. It is still surrounded by the other portions of the macrosporangium, viz. the prothallium or endosperm, the nucellus (if this still persists), and the seed coat formed from the integuments. THE COMPLETE STRUCTURE DERIVED FROM THE OVULE IS TERMED A SEED, AND THE FURTHER DEVELOPMENT OF THE UNOPENED MACROSPORANGIUM TO FORM A SEED

IS CHARACTERISTIC OF ALL PHANEROGAMS. As seed-plants or Spermaphyta the latter may be contrasted with the seedless Cryptogams.

An account of the male sexual generation of the Phanerogams must now be given for comparison with the above short sketch of the development of the female generation.

The MICROSPORES of the Spermaphyta are called POLLEN-GRAINS. They are formed in large numbers within the MICROSPORANGIA or POLLEN-SACS, which are borne singly or in numbers on the MICROSPOROPHYLLS or STAMENS. The part of the stamen which bears the pollen-sacs is usually clearly distinguishable and is called the ANTHER.

The development of the pollen-sac (Fig. 423) commences with divisions parallel to the surface taking place in cells of the hypodermal layer; this separates the cells of the primary archesporium from an outer layer of cells. The latter give rise to three layers of cells the outermost of which is the fibrous layer, the innermost the tapetum, while the intervening layer is later crushed. The archesporium after undergoing a number of divisions forms the pollen-mother-cells, each of which divides as in Pteridophytes into four daughter-cells (cf. Fig. 102). These are the pollen-grains, and are spherical or ellipsoidal in shape and provided with a cell wall; an external cutinised layer (the EXINE), and an inner cellulose layer, rich in pectic substances (the INTINE), can be distinguished in the wall.

While the male sexual cells of all archegoniate plants are dependent on water for their conveyance to the female organs, the transport of the pollen-grains to the egg-cells is brought about in Seed-plants by means of the wind or by animals. However far the reduction of the male prothallium has proceeded-and even in the case of the heterosporous Pteridophyta only a single sterile cell was present-two constituent parts are always distinguishable in the germinating pollengrain; these are a VEGETATIVE CELL which grows out as the POLLENTUBE, and an ANTHERIDIAL MOTHER-CELL which ultimately gives rise

to two GENERATIVE CELLS. The pollen-tube, the wall of which is continuous with the intine of the pollen-grain, ruptures the exine and penetrates, owing to its chemotropic irritability, into the tissue of the macrosporangium (cf. p. 279). The antheridial mother-cell passes into the pollen-tube and sooner or later gives rise to two generative cells which reach the embryo-sac and egg-cell by passing along the pollen-tube. The name Siphonogams has been applied to the seed-plants on account of the common character of the group afforded by the formation of a pollen-tube.

The results reached by the above survey may be summarised by

FIG. 423.-Hemerocallis fulva. A, Transverse section of an almost ripe anther, showing the loculi ruptured in cutting; p, partition wall between the loculi; a, groove in connective; f, vascular bundle (× 14); B, transverse section of young anther (× 28); C, part of transverse section of a pollen-sac; pm, pollen-mother-cells; t, tapetal layer, later undergoing dissolution; c, intermediate parietal layer, becoming ultimately compressed and disorganised; ƒ, parietal layer of eventually fibrous cells; e, epidermis (× 240); D and E, pollen-mother-cells after division (x 240).

saying that the Phanerogams continue the series of the Archegoniatae and agree with the latter in exhibiting an alternation of generations. While the asexual generation becomes more complex in form and more highly organised, there is a corresponding reduction of the sexual generation. The female sexual generation is enclosed throughout its whole development in the asexual plant, and only becomes separated from the latter in the seed, which further contains as the embryo the commencement of the succeeding asexual generation. The exhaustive investigations made of recent years into the phenomena of the reduction division (cf. p. 84) in the spore-mother-cells of archegoniates and Phanerogams have resulted in a confirmation of the limits of the two generations in the latter (2a). The number of chromosomes char

primary
Prothallium
=Antipodal
cells
and secondary
Prothallium
=Endosperm
Egg appa-
ratus

acteristic of any plant is diminished to one-half, during the divisions that lead to the origin of the sexual generation, and the full number of chromosomes is not again attained until fertilisation takes place. The asexual generation has always the double number, the sexual generation the single number of chromosomes. The gametophyte is haploid, the sporophyte diploid (cf. p. 95).

The Spermaphyta are divided into two classes (3) which differ in their whole construction: (1) the Gymnosperms, with naked seeds; (2) the Angiosperms, with seeds enclosed in an ovary.

The names of these classes indicate the nature of one of the most important differences between them. THE CARPELS OF THE ANGIO

SPERMS FORM A CLOSED CAVITY, THE OVARY, WITHIN WHICH THE OVULES DEVELOP. SUCH AN OVARY IS WANTING IN THE GYMNOSPERMS, THE OVULES OF WHICH ARE BORNE FREELY EXPOSED ON THE MACROSPOROPHYLLS OR CARPELS.

The Gymnosperms are the phylogenetically older group. Their construction is simpler and in the relations of their sexual generation they connect directly with the heterosporous Archegoniatae; they might indeed be perhaps best treated as belonging to this group.

The Angiosperms exhibit a much wider range in their morphological and anatomical structure. The course of their life-history differs considerably from that of the Gymnosperms, and without the intermediate links supplied by the latter group, the correspondence with the life-history of the Archegoniatae would not be so clearly recognisable.

These conclusions are confirmed by the evidence afforded by Palaeobotany. Gymnosperms or forms resembling them are found along with what appear to be intermediate forms between the Gymnosperms and the Pteridophyta in the fossiliferous rocks of the Devonian, Carboniferous, and Permian formations. The Angiosperms are, on the other hand, first known from the Cretaceous formation.

Morphology and Oecology of the Phanerogamic Flower and of its Sexual Generation

The Phanerogamic Flower (4)

The distinction between the gymnospermic and angiospermic plants is briefly and well expressed by the statement that the seeds of the Gymnosperms are exposed while those of the Angiosperms are enclosed. Only an accurate examination of the floral features, especially of the structure, equipment, and development of the sexual generation in the two cases will enable us to form a correct judgment on the differences between these two great classes of Phanerogams.

The flowers of the Gymnosperms are all unisexual and diclinous.

The macrosporophylls form the female, the microsporophylls the male flowers. The two sexes are found either on the same individual (MONOECIOUS), or each plant bears either male or female flowers (DIOECIOUS). Leaves forming an envelope around the group of sporophylls are only found in a few flowers of the Gymnospermae. (Gnetaceae).

The MALE FLOWERS are shoots of limited length, the axis of which bears the closely crowded and usually spirally arranged sporophylls. The scales which invested the flower in the bud often persist at the base of the axis (Fig. 424). The microsporangia are borne on the

FIG. 424.-Pinus montana. A, Longitudinal section of a ripe male flower (x 10). B, Longitudinal section of a single stamen (x 20). C, Transverse section of a stamen (x 27). D, a ripe pollengrain of Pinus silvestris (x 400).

lower surface of the sporophylls, two or more being present on each. Their opening is determined as in the sporangia of the Pteridophyta by the peculiar construction of the outer layer of cells of the wall (exothecium). The pollen-grains are spherical, and are frequently provided with two sacs filled with air which increase their buoyancy and assist in their distribution by the wind (Fig. 424). On germination the outer firm layer of the wall of the pollen grain (exine) is completely lost, being fractured by the increase in size of the protoplasmic body (cf. Fig. 459 D).

In many Gymnosperms the FEMALE FLOWERS or CONES resemble the male flowers in being composed of an axis bearing numerous spirally arranged sporophylls. In other cases they differ from this