142 DISPERSION OF POLLEN BY THE WIND. 



tered, all in the space of a few minutes. The earliest discharge of pollen begins 

 between 4 and 5 a.m. in the height of summer, and the plants which take part 

 in it thus early are the Meadow-grass (Poa), Kcderia, and Avena elatior. A 

 little later, between 5 and 6 o'clock, comes the turn of the Quaking-grass (Briza 

 media) and Aira ccespitosa, and of Wheat and Barley (Triticum, Hordeum). 

 Between 6 and 7 polHnation occurs in Rye and in a great number of different 

 Grasses which grow in meadows, such as Cock's-foot-grass (Dactylis), Atidro- 

 pogon, the Brome-grasses (Brachypodium), and many species of Fescue (Festuca). 

 Between 7 and 8 o'clock the pollen is liberated from Oats of the Trisetvm, 

 group, from the Fox-tail-grass {Alopecurus), Timothy Grass (Phleum), and the 

 Sweet Vernal Grass {Athoxanthum). An interval now intervenes, at least 

 amongst the indigenous Grasses. Of exotic species which are cultivated in 

 gardens the following discharge their pollen in the course of the forenoon, viz. 

 the Millets (Panicum imilliaceum and Sorghum) between 8 and 9 o'clock; Setaria 

 Italica and the Brazilian Pampas-grass (Oynerium argenteum) between 9 and 

 10 o'clock. Towards noon indigenous Grasses come again into play. About 11 

 o'clock pollination takes place in most species of the Bent-grass genus (Agrostis), 

 and between 12 and 1 in Melic-grass (Melica), MoUnia, Mat-grass (JSfardus), 

 ElyTnus, Sclerochloa, and several species of Galamagrostis. In the course of 

 the afternoon the process takes place in a few isolated species, as, for instance, 

 in some Brome-grasses at 2 o'clock, in a few species of Oat {Avena) at 3, 

 in Agropyrwm, at 4, and in Aira fiexuosa between 5 and 6. It is worthy of 

 note that the Soft-grass (Holcus), under favourable atmospheric conditions, 

 opens its glumes, pushes forth its anthers, and liberates pollen twice a day, 

 once in the morning at about 6 o'clock, and a second time in the evening 

 at about 7 — provided always that the temperature of the air is not less than 

 14° C. The entire process lasts in most cases from 15 to 20 minutes for each 

 flower. 



With the opening back of the glumes and extrusion of the anthers are often 

 connected alterations also in the position and inclination of the stalks which bear 

 the spikelets. For example, the pedicels of the spikelets of Agrostis, Apera, 

 Galamagrostis, Koderia, and Trisetum divaricate from the axis, so as to form with 

 it angles of from 45° to 80° for the period of pollination. But as soon as the poUen 

 is discharged all these stalks move back towards the main axis of the inflor- 

 escence, and the panicle, as it were, contracts. These movements are obviously 

 designed to give sufficient room to the anthers when they are exserted, in order 

 that they may oscillate freely and so disperse their pollen. In those Grasses where 

 the flowers are crowded together in close spikes, and also in the large Carex section 

 of the Oyperacese, the bracts do not spring open but only relax, and sometimes 

 merely to such a slight extent that it is scarcely noticeable on cursory inspection. 

 The thread-like filaments are also only partially visible in cases of the kind, the 

 anthers are pushed forward and raised above the glumes through the rapid growth 

 of their filaments. As soon as a filament reaches the proper length its upper 



