Division 5. =Angiospermæ.= The carpels surround the ovules and form an entirely closed chamber (_ovary_), in which the ovules mature and ripen into seeds. The surface of a portion of the apex of the carpel is transformed into the “stigma,” which, by a sticky fluid and also by hair-structures, is capable of retaining the pollen-grains conveyed to it by the wind, or more frequently by insects. The pollen-tube grows from the stigma, through the “conducting cellular tissue” (_style_), to the ovules. The pollen-grains contain two cells, a vegetative and a free generative cell. The latter passes into the pollen-tube and there divides into two, one of which is the sperm-nucleus. The female prothallium, which is intended to serve as nutritive-tissue, is formed _after fertilisation_. Archegonia are wanting. DIVISION IV. GYMNOSPERMÆ. The following characters should be added to those already given on page 2:-- The Gymnosperms comprise only trees or shrubs. The flowers are always _unisexual_ and destitute of perianth (except _Gnetaceæ_); the female plant of _Cycas_ is the only one which has no flower. The MALE FLOWERS are constructed on the same type as the cones of the Horsetails and Club-Mosses, and are _most frequently long shoots_ (Figs. 243, 258, 260 _A_, 267 _J_) bearing a number of spiral or verticillate stamens. The FEMALE FLOWERS are of a more varied structure (see the orders). The OVULE _is orthotropous_ (except _Podocarpus_ which is anatropous) and projects from the carpel uprightly, inverted, or horizontally; it has usually _only one integument_ (compare however Taxaceæ) which proceeds from the upper part of the nucellus, so that the embryo-sac in part is placed below the integuments (Figs. 251, 264). The drop of mucilage which catches the pollen-grains dries up and draws the pollen-grain through the micropyle to a space just above the nucellus--_the pollen-chamber_--in which the germination of the pollen-grain commences. In each seed, only one of the many embryos which are formed proceeds to its full development. The seed is always _endospermous_, and the embryo has one, two, or a whorl of several cotyledons. A vigorous primary root is developed on germination. THE VASCULAR BUNDLES in the stem are arranged in a ring, and _increase in thickness_ takes place by a closed cambium-ring which forms bast (_phlœem_) externally, and wood (_xylem_) internally with distinct annual rings, _as in the Dicotyledons_. Only certain of the Cycadeæ deviate from this arrangement. The _secondary wood_ is very uniform, as it is almost exclusively _formed of tracheides_ with bordered pits, but _true vessels are wanting_; this also indicates a relationship with the Pteridophyta (see page 202). The Gymnosperms are biologically lower than the Angiosperms; they are wind-fertilised, and without extra floral-nectaries.