STAMENS 361 



length of the anther. Each archesporial cell divides tangentially 

 (Fig. 209, C), and, from the inner halves (a.) thus formed, the 

 actual pollen-producing tissue arises, so that the pollen mother- 

 cells are really subepidermal in origin. The outer half of each 

 archesporial cell divides to form the wall of the pollen sac 

 (Fig. 209, D), which comes to consist of several layers. 



In transverse sections through older anthers (Fig. 209, A ; 

 Frontispiece, Fig. 210), the mass of spore mother-cells in the 

 centre of each pollen sac is seen to be surrounded by a nutritive 

 layer (tapetum, t.). This is composed of large, often palisade- 

 like, cells with prominent nuclei and dense contents. Between 

 the tapetum (t.) and the epidermis (ep.) of the anther are two 

 or more layers, the outermost of which generally consists of 

 rather large, subsequently thickened cells (fibrous layer, /.), 

 whilst the others (m.l.) are usually very much flattened. 



Each pollen mother-cell, as in the groups previously studied, 

 undergoes two divisions (cf. Fig. 209, A), the first of which is 

 the reduction division. The four pollen grains, thus formed 

 within each mother-cell, generally fall apart as the membrane 

 of the latter becomes dissolved. But in a few cases the members 

 of each tetrad remain in connection, when the pollen is shed 

 (e.g. Typha, Neottia, Fig. 211, D, a-c ; Ericaceae, Fig. 211, D, d.), 

 and a more extreme condition is seen in Orchids, where all the 

 pollen grains formed by the single stamen cohere as two masses 

 or pollinia (Fig. 211, F). Such cohesion is, however, only met 

 with in insect-pollinated flowers. During the development of 

 the pollen grains the tapetum undergoes gradual disorganisation 

 (Fig. 212, t.), prior to which the cells often become bi- or 

 multi-nucleate. 



The mature pollen grain possesses a, wall of two layers, the 

 outer of which is thick and cuticularised, whilst the inner is 

 delicate. Where the pollen is wind-borne, its surface is com- 

 monly smooth, 1 but when conveyed by insects it is usually 

 sculptured in various ways (Fig. 211, B, G). The outer layer 



1 Similar smooth pollen is characteristic of plants in which pollination 

 is effected under water (e.g. Hornwort). Such water-borne pollen is some- 

 times highly specialised, that of the Sea-Grass (Zostera), for example, being 

 tubular, of the same specific gravity as sea-water, and without any cuticu*- 

 larised layer, 



