140 DISPEESION OF POLLEN BY THE WIND. 



consequence discharged in little pinches as though from a sugar-sifter. If the 

 flowers of this kind of plant contain pistils as well as stamens, the relative 

 development of the two sets of organs is always so regulated that the stigmas 

 are already perfect and adapted to the reception of pollen at a time when the 

 anthers of the same flowers are still hidden beneath the floral or iuvolucral 

 envelopes and the pollen is consequently immature. By the time the pollen is 

 completely developed and is in a state to be discharged from the opened anthers, 

 the stigmas of the flower in question are withered and are no longer capable 

 of taking up the pollen. Hence it follows that in these plants the pollen-dust 

 must be transported to other flowers which happen to be at a younger stage of 

 development if fertihzation is to be brought about. This is what occurs in 

 nature through the instrumentahty of gentle breezes which impose a tremulous 

 motion upon the anthers. 



In the first rank of plants belonging to the above category stand the 

 Grasses. Their mode of pollination is so remarkable that it is worth while to 

 look into it a little more closely. One group of Grasses — of which Avena elatior, 

 represented in fig. 231, is an excellent example — commences the process under 

 discussion by a sudden distension of the bracts (known by the name of glumes) 

 through the instrumentality of a special turgid tissue situated at their base. 

 The result is that the anthers, till then concealed, are exposed, and it becomes 

 possible for them to be exserted beyond the glumes into the air. This exsertion 

 is effected by an extraordinarily rapid longitudinal growth on the part of the 

 filaments. It has been calculated that in some grasses the filiform filaments 

 elongate to the extent of 1-1 '5 mm. in the course of a minute, and that usually 

 in ten minutes they are three or four times as long as they were originally. 

 In one subsection of these plants the filaments grow downwards, in another 

 horizontally, and in a third straight upwards towards the sky. The turgidity 

 of the cells in these delicate filaments is so great as to enable even those which 

 grow vertically upwards to support the weight of the anthers without bending. 

 In the case of those Grasses whose stamens grow downwards from the 

 beginning it does no doubt look as though this direction were assumed in 

 consequence of the weight of the anthers. This is not, however, the fact. A 

 high degree of turgidity exists here also, and if one inverts the inflorescences 

 of this kind of Grass, the stamens which have just completed their longitudinal 

 growth remain quite stiff", in spite of their extreme slenderness, and project 

 straight up. Soon after, it is true, this condition ceases. The fllaments become 

 slack; those that were erect nod and droop, those that were horizontal fall 

 down, and the anthers are then all suspended at the ends of oscillating threads. 



The dehiscence of the anthers is accomplished synchronously, with these 

 changes in the filaments. As long as the anthers lay hidden beneath and pro- 

 tected by the glumes they were straight and linear in form (see fig. 231 ^). Each 

 anther consists of two contiguous parallel lobes, and each lobe has a line running 

 longitudinally down it, along which dehiscence takes place. This operation 



