14.6 DISPERSION OF POLLEN BY THE WIND. 



membranous scale iu which the connective terminates being turned up, whilst 

 on either side of the middle line there is a shallow trough (fig. 233 '). It is easy to 

 convince one's self of the fact that these excavations serve for the reception of the 

 pollen which falls from the anthers above them (see fig. 233^), and inasmuch as all 

 the anthers crowded together in a particular spike usually open simultaneously, all 

 the scales of the spike in question have their backs covered with the pollen-dust 

 at the same time (fig. 233^). As long as there is no wind the pollen rests on the 

 scales where it has been deposited; but the moment a gust of wind shakes the 

 boughs and twigs of the Pine-tree the pollen is removed from its resting-place, 

 and whole clouds of yellow dust may be seen being blown upward from the spikes 

 (c/. fig. 233*). 



The corresponding adjustment in the Yew (Taxus) differs to a certain extent 

 from the above which is so characteristic of Pines and Firs. The connective of the 

 anther-lobes in Taxus does not terminate in an upturned scale, but in a little 

 circular shield with an crenate margin. The anther-lobes are attached to the under 

 surface or back of this shield (see fig. 234 '). Moreover, the anthers are united into 

 roundish heads, and the peltate connectives lock closely together like the separate 

 parts of a mosaic, so that the pollen-sacs are not visible superficially. When the 

 pollen has reached maturity and has assumed the form of powder, the pollen-sacs 

 concealed underneath the shields burst open, their walls shrivel, and the anthers 

 have then the appearance portrayed in fig. 234 \ The shields now resemble cupolas, 

 supported by short columns, and arching over spaces in which is stored a heap of 

 loose powdery pollen. Under the influence of a warm, dry atmosphere the tissues 

 of the shields contract somewhat, and in consequence chinks appear between the 

 shields, and the spherical conglomeration of anthers seems to be rent asunder (see 

 fig. 234 ^). When the branches of the Yew are caused to sway by a gust of wind a 

 portion of the pollen is at once blown out through these chinks in the form of a 

 little cloud. In the evening when the atmosphere becomes damper, as also on dull 

 and rainy days, the shields lock together again, and such pollen as is left is once 

 more inclosed and protected from wet. On the return of warm, dry weather the 

 cracks reappear, and the remainder of the pollen may be shaken out and blown 

 away. 



The mechanism which has here been described in the case of the Yew, that 

 being an easily accessible example, is found to exist in its main feature, though with 

 many varieties of detail, in the Juniper, in Cypress, and Arbor VitiB (Juniperus, 

 Cupressus, Thuja). One species of Juniper, viz.: Juniperus Virginiana, in which 

 the little heads of stamens are closed when the atmosphere is damp, and open when 

 it is dry, has been already selected for illustration in figs. 226 '^' ^^^ ''• !»■ p. 125. 

 Curiously enough, the Planes (Platanus), which are not related to the Conifers 

 just referred to, exhibit similar characteristics in the matter of pollen liberation. 

 The stamens have a peltate or pulvinate connective spread out over the anthers, 

 and each stamen, considered by itself, resembles a short peg or nail with a large, 

 thick head. Besides bearing little papillae, which are looked upon as abortive 



