The fertile leaves or sporophylls are generally aggregated on special shoots to form rioweN which may contain one or both kinds The microspores are set free from the sporangiurn and carried generally by wind or insect agency to the vicinity of the macrospore, which never leaves the ovule.
These, as in Gymnosperms, are of two kinds, microspores or pollen-grains, borne in the stamens (or microsporophylls) and megaspores, in which the egg-cell is developed, contained in the ovule, which is borne enclosed in the carpel (or megasporophyll).
The spores produced in each sporangium vary from very many to a single one in the case of some heterosporous forms. These latter bear spores of two kinds, microspores and megaspores, in separate sporangia.
Some Calamites were heterosporous, sporangia with microspores and megaspores being found in the same cone.
On germination the microspores give rise to a reduced prothallus, consisting of the small cell first cut off and a wall of cells enclosing two to four central ones; "from these latter the biciliate spermatozoids originate.
The microspores on germination produce a small, greatly reduced male prothallus bearing one or two antheridia which give rise to a number of spirally coiled, multiciliate spermatozoids.
The microspores are united by means of hardened protoplasm into one or more masses, while the solitary megaspores have a more or less complicated episporium.
These are with few exceptions foliar structures, known in comparative morphology as sporophylls, because they bear the spores, namely, the microspores or pollen-grains which are developed in the microsporangia or pollen-sacs, and the megaspore, which is contained in the ovule or megasporangium.
25, f), supporting at its summit the anther (a), consisting of the pollen-sacs which contain the powdery pollen (p), the microspores, which is ultimately discharged therefrom.
The pollen-grains or microspores contained in the anther consist of small cells, which are developed in the large thick-walled mother-cells formed in the interior of the pollen-sacs (microsporangia) of the young anther.
And contain either an im The upper sporangia contain numermense number of minute ous microspores; in each of the lower spores or a very small number sporangia four megaspores are shown.
The spores of this genus are curiously winged, and intermediate in size between the microspores and megaspores of Lepidostrobus; the question of homospory or heterospory is not yet decided.
The microspores are tetrahedral.
8, Bennettites, synangium of male flower, showing line of dehiscence, k, and microspores, 1.
9, Synangium, in transverse section, showing sporangial groups, m, and microspores, 1.