ANGIOSPERMS Details of flowering-plant pollination mechanisms are given by Mari laun (1895), Knuth (1906), Erdtman (1943, 1957), Wodehouse (1945) and others.
(i) Grasses, rushes, sedges and their allies. The Gramineae, Cyperaceae, Typhaceae, and Juncaceae are typically wind-pollinated. From the raised inflorescences of grasses, the anthers are extruded on long filaments to which they are so lightly attached that they vibrate in the slightest wind. Often, as in the end of each pollen-sac bends up (Fig. 5), forming a spoon into which pollen is shed from a slit, and where it ac cumulates until blown away by the wind. Either damp or very dry weather may delay both extrusion of the stamens and splitting of the anthers. Except for rye and maize, most cultivated cereals are self-fertilized and shed little pollen, but pasture grasses are free shedders.
In central Europe, Marilaun (1895) found that different grasses flowered for brief periods of only 15 to zo minutes daily, and at charac teristic times of the day: This timetable does not necessarily apply elsewhere, and, in Nebraska, Jones & Newell (1946) found a less precise timing and showed that anthesis is determined by temperature. They distinguish cool-season from warm-season grasses. Cool-season grasses include: Festuca elatior (anthesis at 13.30-15.00 hours); Agropyrurn spp. (14.00-18.3o hours); inermis (14.30-19.00 hours); Poa pratensis (during the night); Secale cereale (o2.30-11.30 hours, maximum 06.00-08.30 hours). The warm-season group includes: Bouteloua gracilis (o3.oa-o9.00 hours, maxi mum 04.30-05.30 hours during darkness); Buchloe dactyloides (o6.30-13.00 hours, maximum 07.00-08.30 hours); Panicum virgatum (10.00-12.00 hours, delayed in cool season); Zea mays (07.30-I6.00 hours, maximum 08.30-11.00 hours).
Hyde & Williams (1945, p. 89), from the cooler climate of Wales, report both discrepancies and agreements with Marilaun's timetable: Holcus lanatus (04.00-06.00 hours, but mainly at 14.00-19.00 hours); Cynosurus cristatus (05.00-06.00 hours); Arrhenatherlma (07.00-08.00 hours); Trisetrnn flavescens (before o8.00 hours); Festuca pratensis (o8.00 14.0o hours).
(ii) Aquatic monocotyledonous herbs include a few other wind-pollinated plants, for example Triglochin and Sparganium, while in the genus Potamogeton some species are pollinated by wind and others by water.
(iii) Entomophilous herbs and low shrubs include some species in which the phase of insect visitation is followed by an opportunity for wind pollination, the anthers first shedding pollen within the corolla; but as the flower matures, the elongating filaments protrude and scatter pollen in the wind. These types include the semi-parasites Bartsia and Lathraea (Rhinanthaceae), and the heaths Calluna and Erica—but not Rhododendron, which has very sticky pollen.
(iv) Tropical and subtropical trees include few anemophilous species, but Casuarina and Myrothazzzrnus are wind-pollinated, and some of the palms, although entomophilous, shed a good deal of pollen, which may be carried by the wind.
(v) Nettles and their allies form an anemophilous group which do not store pollen after dehiscence of the anthers. The anthers dry as they mature, tissue tensions are set up, and suddenly, as the pollen sacs burst, the filaments uncoil, throwing pollen into the air. The process can be watched on a still, dry day when small puffs of pollen appear as the nettle flowers explode, but in damp air dehiscence of the anthers is inhibited. The mechanism occurs in Urtica, Parietaria, Mortis, and Broussonetia. A sifter mechanism similar to that of the grasses occurs in Cannabis and Humu/us.
(vi) Herbs with inflorescences elevated above the general level of the foliage include a number of anemophilous types such as Mercurialis. In Plantago and Globularia the anthers, which are exposed in cups, close their slits in moist weather but shed their pollen in dry air. Upward facing cups occur also in Poterium and Sauguisorba. Sifter mechanisms occur in some species of Rumex and Thalictruna. Other conspicuous pollen shedders occur in the Chenopodiaceae (Beta, Salsoffa, Cfienopodiuia) and in the Amaranthaccae, and also in some groups within the Compositae especially Ambrosia and Artennisia.
(vii) Deciduous trees of temperate regions form a biological group. Typically the male flowers are aggregated into pendulous catkins, usually appearing shortly before the leaves expand. In Alnus, Betula, Castanea, Corylus, Fagus, 7uglans, Populus (Salix, like T ilia , is both insect-pollinated and a wind shedder), and Qtercus, pollen is protected from rain after shedding while temporarily stored on the upper scales of the flower standing underneath—until it is blown away by wind in a manner reminis cent of Pizzas. Platanus closes its catkins by a hygroscopic mechanism, so that pollen is not merely protected from rain but can be blown away only in dry weather. Hippophae pollen is shed into the base of the flower while this is still in bud. At maturity the perianth lobes remain united at the top but separate at the base, leaving slits through which pollen can be removed by the wind. In another group, including Fraxinus, Bunts, Phillyrea and Ulznus, the anthers project as upward-facing cups from which pollen is removed by wind.
The take-off mechanisms briefly sketched in this Chapter, with others (doubtless including some still undiscovered), are not mere curiosities of natural history. On the contrary, they are highly efficient processes that restrict spore liberation to limited meteorological conditions. Pollen and spores of mosses and ferns tend to be shed into dry winds. Ascomycetes and lower Basidiomycetes are more likely to discharge spores when the substratum is wet. Spore shedding in higher Basidiornycetes is less affected by air humidity and wind speed. Spores of some Fungi Imperfecti may depend on wind for removal, or on changes in humidity for hygro scopic movements, or on rain for splash dispersal. Soil- and dust-borne bacteria and protozoa are probably borne aloft in high winds from heated or mechanically disturbed ground. The nature of the `take-off' mech anism profoundly affects the occurrence of different kinds of spores or pollens in the atmosphere—with consequent significance for hay fever patients, seed-crops, plant diseases, evolution, and geographical distribution.