344 THE BIOLOGY OF FLOWERING PLANTS 



indeed it could not function. In the Ericaceae, where the 

 endothecium is much reduced, dehiscence occurs by pores 

 instead of by slits. These pores, two in number, open at 

 the apex of the anther by the dissolution of a special tissue. 

 In some cases, e.g. Rhododendron, the pollen is squeezed 

 out by the contraction of the anther ; in others, e.g. Calluna, 

 it lies in the anther and is shaken out by visiting insects. 

 In the Berberidaceas, Lauraceae, and some other families, 

 dehiscence is again by pores which open by the shrinkage 

 of valve-like lids, to which, in these cases, the endothecium 

 is confined. The sticky pollen is carried out attached to 

 the Ud. 



Pollen is sometimes shaken out of the anthers, especially 

 in wind-pollinated plants. Goebel states that only in 

 Ricinus is this due to a mechanism of the anther. In Ricinus 

 the thickening of the walls of the exothecial cells is different 

 near the future opening and on the opposed side of the 

 anther. In the former the walls are spirally and annularly 

 thickened, and tend to resist the opening movement of the 

 latter ; when dehiscence actually occurs the resistance is 

 suddenly released and the wall flies violently back, throwing 

 out the pollen in a cloud. 



In all other cases the sudden movement which ejects the 

 pollen is not connected with the opening of the anther, but 

 is due to a tension in the filament. The most famous case 

 is that of Parietaria officinalis, the wall pellitory, in which 

 the elastic filaments are held arched in by the anthers sticking 

 in the boat-shaped perianth segments. At a certain stage 

 of the opening of the male flower, often as the result of a 

 slight shock to the plant, the anthers come free with a jerk ; 

 they fly back and the pollen explodes into a little puff. 



The Pollen. — The pollen grains vary enormously in 

 shape, size, and other characters. In size they range from 

 0*0025 mm. in diameter in Myosotis to 0*25 mm. in Cucur- 

 bita. They are usually round or oval, but may have sharp 

 angles. Like fern spores, they possess a delicate inner wall, 

 the intine, and a stout outer wall, the extine, in which thin 

 spots are often found, through which the pollen tube, an 



