The ovule is usually surrounded by one integument, which projects beyond the tip of the nucellus as a wide-open lobed funnel, which at the time of pollination folds inwards, and so assists in bringing the pollen-grains on to the nucellus.
Gazette, xlvi., 1908, regards this tissue as belonging to the nucellus.) At the time of pollination the long tubular integument secretes a drop of fluid at its apex, which holds the pollen-grains, brought by the wind, or possibly to some extent by insect agency, and by evaporation these are drawn on to the top of the nucellus, where partial disorganization of the cells has given rise to an irregular pollen-chamber (fig.
Cross pollination is often favoured by dimorphism of the flower, as shown in species of Primula (fig.
After pollination the pollentube grows into the nucellar tissue, as in cycads, and the pollen-grain itself (fig.
In the case of a dimorphic flower, such as Primula, four modes of pollination are possible, two distinguished by Darwin as legitimate, between anthers and stigmas on corresponding levels, and two so-called illegitimate unions, between anthers and stigmas at different levels (cf.
Think of the possibilities if we could reduce pollination of weeds and intrusive vegetation.
The pollination, of flowers and the dispersal of seeds by various animals are biological factors; but pollination and dispersal by the wind cannot be so regarded.
Pitfall flowers such as Asarum, Aristolochia and Arum maculatum, when the insect is caught and detained until pollination is effected (fig.
Humming-birds and honeysuckers are agents of pollination in certain tropical plants; they visit the generally large and brightly-coloured flowers either for the honey which is secreted in considerable quantity or for the insects which have been attracted by the honey (fig.
In function the perianth may be compared with a unilocular ovary containing a single ovule; the projecting integument, which at the time of pollination secretes a drop of liquid, serves the same purpose as the style and stigma of an angiosperm.