136 DISPERSION OF POLLEN BY THE WIND. 



from the branches in the form of swinging tassels whilst the flowers containing 

 mature stigmas are always above them, whether situated on the same or on 

 adjoining branches (c/. fig. 228). In Fir-trees, only the pendent lateral branches 

 of the boughs bear the male inflorescences, which at a distance look almost like 

 red mountain-sti-aw berries, whilst the female inflorescences stand up in the form 

 of little cones on the top of the same boughs like tapers on a Christmas-tree; 

 indeed, many Fir-trees bear the female flowers only on the highest branches close 

 to the summit, and on the lower bouo:hs none but male flowers, and under such 

 circumstances pollen could not possibly reach the stigmas if it were only carried 

 by the wind in a horizontal direction. Even in dicecious plants {i.e. where the 

 male and female flowers are on distinct individuals) this relatively inferior 

 situation of the staminal flowers is often to be observed, the end being attained 

 by the fact that the individuals bearing male flowers grow less high than those 

 bearing female flowers. Thus, for example, in Hemp-fields one may see that the 

 plants discharging pollen never reach the same height as those whose flowers are 

 to receive the pollen. Exceptions to the rule do, it is true, appear to exist in 

 the Bulrush {Typlia), the Bur- reed (Sparganium), and many species of Sedge 

 (Carex), which possess monoecious flowers, inasmuch as in them the male flowers 

 are situated above the female; but in consequence of the non-simultaneous elon- 

 gation of the axis, it usually comes about that the mature female flowers of a 

 plant whose stem is amongst the older and taller ones rests at a higher level 

 than the male flowers of the individual next to it whose stem is younger and 

 shorter, and it is easy to convince one's self by observation that here also the 

 pollen is not conveyed by the wind in a horizontal direction but obliquely 

 upwards, and is wafted to the stigmas of neighbouring plants. 



This must not, of course, be looked upon as implying that when pollen is 

 dispersed by the wind none descends; but it is unquestionably true in the majority 

 of cases that the clouds of pollen which are carried off" by moderate winds at first 

 soar upwards and either reach the stigmas awaiting them at a higher level direct 

 in their way, or else, later on, when the air is still and the pollen-cells are scattered 

 over a wider space, they sink slowly down, leaving a deposit on the stigmas, 

 just as when dust is raised in a room it ends by slowly falling again and covering 

 the furniture with a uniform layer. 



In some species at the very moment when the anthers burst open the pollen 

 is ejected violently into the air and ascends obliquely in the form of a little 

 cloud of dust. In this country a good example of this phenomenon is aflbrded 

 by the Nettles. Anyone standing in front of a bed of Stinging Nettles on a bright 

 summer morning, and waiting until the fii"st rays of sunshine fall on the flowers, 

 will be surprised to see small pale-coloured clouds of dust ascending here and there 

 from amidst the dark foliage. At first the clouds are solitary, and are given off" at 

 measurable intervals; by degrees they become more frequent, and at times one may 

 see five or six or more arising at the same moment and at no great distance from 

 one another. But gradually the little explosions become less frequent again, and 



