POLLEN. 95 



the pollen is temporarily stored in some portion of the flower and preserved for 

 future use. 



More frequently, however, the pollen remains within the opened anther. 

 Usually these flowers are visited by insects which disturb the anthers and release 

 the pollen, or they dust themselves over with it and carry it ofl' to another 

 flower. 



The fact that the anthers are directed sometimes inwards, sometimes outwards, 

 is correlated with these insect-visits. Where the slits or pores of the anthers are 

 directed towards the periphery of the flower, one speaks of outwardly-directed 

 anthers (extrorse), where toward the centre of the flower, of inwardly-directed 

 (introrse). These relations are of importance in respect of insect-visits. If the 

 honey is situated outside the whorl of stamens, the insects must pass between 

 the stamens and petals to secure the nectar, as in Colchicuvi, Iris, Convolvulus, 

 Epimediwm, and Laurus. Here it will be advantageous for the anthers to be 

 extrorse. On the contrary, when the honey is between the ovary and the bases of 

 the stamens, and the insects have to penetrate to this region, as in Gentians and 

 Opuntias, the stamens will be introrse. It is of great importance that the pollen 

 exposed in the anthers should be rubbed oflf by the insects and carried to other 

 floM^ers, a result only obtainable when the dehiscent side of the anther is placed 

 in the way of the insect as it enters or leaves the flower. 



Numerous other peculiarities affecting the structure, position, and movements of 

 stamens will be dealt with later on, when treating of the removal of pollen from 

 and to flowers by insects and other animals. 



POLLEN. 



Like all other leaf-structures, stamens arise in the first instance as convex 

 projections from their points of insertion on the stem. These projections consist 

 of a homogeneous, small-celled tissue. They soon, however, assume a club-shaped 

 form, and the outlines of anther and filament become recognizable. A vascular 

 bundle is found traversing the entire length of each stamen, and the anther, which 

 increases in size more rapidly than the filament, shows symmetrically-arranged, 

 longitudinal grooves, with projecting portions between, an-anged in pairs. The 

 cells situated immediately below the surface of the young anthers become now 

 marked out into tissues of two kinds. Towards the outside three layers of cells 

 become distinguishable, and these, with the outermost, enveloping layer give rise to 

 the wall of the anther; within, large cells become conspicuous, and form what is 

 known as the archesporium. 



These archesporial cells are arranged either in nests or in longitudinal rows 

 embedded in the surrounding tissue. In the latter, the more usual, case, there are 

 four, rarely two or eight, such rows arranged in pairs right and left of the central 

 vascular bundle. Although at this stage of development all the cells of the anther 

 hang together into a continuous mass, the existence of the future pollen-sacs — now 



