138 



DISPERSION OF POLLEN BY THE WIND. 



upon it. The filaments are very thick and, in the closed bud, are tucked in (229^): 

 they are in a state of tension like a spring, but as soon as the cup-shaped perianth 

 opens the filaments spring up one after another, whilst at the same instant the 

 anther-cavities burst open and the pollen is ejected with force into the air (229*). 

 When all the anthers are empty the filaments curve backwards (229^), and soon 

 afterwards the entire spike of flowers drops off the axis, it being no longer of any 

 value to the plant. 



In all these plants ejection of the pollen only ensues when a light, dry wind 



blows at sunrise and causes an altera- 

 tion in the tension of the tissues con- 

 cerned. If there is no wind at all, or 

 the air is close and damp, or if it rains, 

 the opening of the flowers and ejection 

 of the pollen do not take place, or rather 

 they are postponed until the atmo- 

 sphere has become dry again and a 

 breeze arises which causes the flowering 

 branches to sway about. The results 

 of actual observation are of importance 

 to a proper understanding of the dis- 

 persal of pollen-dust. For it thus ap- 

 pears that the air in motion has to 

 start two processes which supplement ' 

 one another, and must operate in rapid 

 succession if the pollen-dust is to reach 

 the right place and not be lost. The 

 same current of air which causes a 

 liberation and expulsion of the pollen 

 by shaking the flowering axes and by | 

 altering the tension of the tissues of 

 the flowers, also carries the pollen away 

 from the spot where it has been pro- 

 duced and conveys it to its destined goal; and this statement applies to the full 

 extent not only to the case of resilient stamens, but also to all other instances of 

 anemophilous pollination where the pollen is in the form of dust. 



A similar phenomenon is observed in the case of plants with short, thick 

 filaments and comparatively large anthers filled with pollen of a floury consist- 

 ency. The Phillyrea, the Pistachio-nut (Pistacia), the Box-tree (Buxus), and 

 most Ashes, especially the common Ash (Fraxinus excelsior, see fig. 230), may 

 serve to illustrate this group of plants. The development of the carpels in 

 each flower precedes that of the pollen. At a time when the relatively large , 

 fleshy stigmas stretch out far beyond the limits of the inconspicuous floral I 

 envelope, and are already capable of taking up the pollen, the anthers may be; 



i'l 



Fig. 230.— The Ash {Fraxinus excelsior). 



Small bifurcating branch, the left-hand limb of the fork bear- 

 ing male flowers, the right-hand limb bearing hermaphro- 

 dite flowers. 2 Hermaphrodite flower. 3 Two anthers ; 

 the upper one open, the lower one still closed. i natural 

 size ; 2 8 X 6. 



