POLLINATION 21 



contains more British species than any other genus, and specimens can easily be 

 found. Also, knowing the end in view, the steps by which it has been attained can 

 be better understood. 



The anther, a (PI. B, %. 1), has two cells wliich spUt down the front so that the 

 poUen-mass or polhnium in each cell is visible. Its caudicle (stalk), c, is at first free, 

 but later becomes firmly attached to the cap of the viscidium, d, wlaich has beneath 

 a ball of very adhesive matter. This is immersed in liquid in the rostellum, r, a 

 pouch wliich holds both the viscid discs as Darwin called them. In D the pouch has 

 been pressed down a little so as to show the viscidia, which are otherwise concealed 

 as in B. This is necessary as the viscid matter sets hard like cement on exposure to the 

 air. E in PI. B, fig. i, shows a section of the rostellum. When the bee inserts its 

 proboscis into the spur, its head pushes against the pouch, cunningly placed like a trap 

 in a rabbit run. Being supported on an elastic hinge, the pouch moves back, the balls 

 of viscid matter come in contact with the insect's head, and at once adhere. When 

 it withdraws, it carries off the poUinia standing erect on the adherent discs. The bee 

 works up the spike visiting other flowers, but owing to their erect position the 

 poUinia are merely pushed against the face of the anther, and cannot touch the 

 stigma, J, the dark-shaded space surrounding the pouch in B. Meanwhile the polUnia 

 are executing a downward movement, and by the time the bee reaches another spike 

 of flowers, are pointing forwards horizontafly. When the insect enters a flower on 

 the new spike, theu tips are pushed against the tenaciously sticky stigma, to which 

 some of the packets of pollen adhere, and are held with such strength that the slender 

 tlireads by which they are bound to the pollinia give way and break. F shows these 

 threads when they are extended. It is a pretty experiment to pull the tip of a pollinium, 

 and see how easfly the threads stretch, and observe the packets of poUen. It will 

 also be found that the caudicles themselves stretch to a surprising extent, and spring 

 back when released. The object of tliis elasticity is to prevent the packets being 

 snatched away from the stigma through the sudden jerk of the insect leaving the 

 flower. The pollen-packets left on the stigma are easily visible with a lens, or often 

 with the naked eye. The stigmatic secretion has a marked eflect upon the poUen- 

 packets, wliich soon break up into the component tetrads and grains. These all 

 put forth a pollen-tube, like a seed in a seed-bed, which grows down to the ovary, 

 and enters an ovule by the tiny micropyle, and thus fertflises it by the emission of its' 

 protoplasm. The presence of pollen on the stigma stimulates the ovules, which 

 begin at once to develop, and the ovary swells visibly, so that by the time the poUen- 

 tube reaches the ovule, the latter is ready to receive it. Tliis takes about three weeks, 

 or more, especially with waxy pollinia. 



It is a curious fact that the viscid disc to which the pollinium is fixed is not, as 

 formerly supposed, a part of the poUinium (which is the male organ), but is developed 



