150 DISPERSION OF POLLKN BY THE WIND. 



by the wind as above described both the empty stamens and the perianth-leaves 

 at their bases drop off. Only after this has happened does the upper whorl of 

 perianth -leaves relax; the anthers of the three upper stamens burst open, their 

 pollen glides into the bowl-shaped perianth-leaves below, and exactly the same 

 process is repeated as took place in the case of the superior whorls. 



The case of the Sea-Buckthorn {Rippophae; cf. figs. 220 2.3,4,6^ p. 109), is 

 worth mentioning as a third example of the same nature. The flowers of this 

 shrub are conglomerated in little tufts on the sides of woody branches. Each 

 male flower is composed of four stamens and two opposite concave scales; the 

 latter have their edges in contact, so that they form a little bladder within 

 which the four stamens are concealed. The pollen is of an orange-yellow colour 

 and mealy consistency, and is set free from the anthers at a time when the 

 bladder is still closed. It falls into the cavity, and is there completely sheltered 

 from rain and dew by the overarching scales. When a warm, dry wind sweeps 

 over the shrubs the bladders open by two opposite chinks, and the pollen is 

 blown out from its resting-place in small quantities at a time. In damp weather 

 the two scales close up quickly and protect what remains of the pollen from 

 wet; on the return of dry weather they move apart again, leaving a free passage 

 for the wind, which then carries off the rest of the pollen. This simple 

 mechanism ensures the safety of the pollen in the event of rain, whilst enabling 

 it to reach the stigmas of neighbouring shrubs whenever the external conditions 

 are propitious. 



A close connection exists between these various contrivances to ensure that 

 pollination shall only take place at the best possible moments, and the mainten- 

 ance of a free passage in the direction in which the pollen is to be transported 

 by the wind, and further between these adaptations and the shape of the 

 stigmas devised for the reception of the pollen. It is obvious that no barrier 

 must be interposed in the path of the little clouds of pollen-dust on their 

 journey to the stigmas. If the flowers of the Arrow-grass, of Pondweeds, or 

 Grasses were wrapped ia large foliage-leaves a great part of the pollen would 

 adhere to these leaves and would be as irretrievably wasted as if it had fallen 

 to the ground or into the water. On this account also aU flowers which have 

 their pollen blown out of them by the wind are arranged in spikes and panicles 

 at the upper extremities of the shoots and project freely into the air, but are 

 never clothed with a mass of foliage. Particular attention may be drawn to 

 the fact that a large number of plants wherein the pollen is in the form of 

 dust flower before coming into leaf; that is to say, yield up their pollen to 

 the wind at a time when the green foliage is still folded up in the buds or is 

 just emerging from them. The Sea-Buckthorn, the Alder, the Ash, the Elm, the 

 Hazel, the Birch, and the Aspen all flower and discharge their pollen at a season 

 when the branches are bare of leaves (cf. the illustrations on pp. 109, 135, 138, 

 143, and 147). Were these plants to begin to blossom after the complete develop- 

 ment of their extensive foliage the wind-transport of the pollen would be rendered 



