POLLEN. 95 



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

 uture use. 



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

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

 he pollen, or they dust themselves over with it and carry it off" to another 

 lower. 



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

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

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

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

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

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

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

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

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

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

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

 ixposed in the anthers should be rubbed oS" by the insects and carried to other 

 lowers, a result only obtainable when the dehiscent side of the anther is placed 

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



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

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

 md to flowers by insects and other animals. 



POLLEN. 



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

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

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

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

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

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

 ongitudinal grooves, with projecting portions between, arranged in pairs. The 

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

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

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

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

 mown as the archesporiv/m. 



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

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

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

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

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



