550 



PHANEROGAMS. 



separate grains in the four loculi of the anther, and falls out of the anther after it 

 has opened ; some of the more important exceptions will be mentioned hereafter. 



Immediately after the perianth-leaves, or their innermost whorl, first become 

 visible on the receptacle as roundish protuberances, the rudiments of the stamens 

 make their appearance in a similar manner, but usually obtain a considerable start 

 in growth of the corolla, which not unfrequently remains for a considerable time in 

 a very rudimentary condition. These bodies, which consist of homogeneous primary 

 meristem, very soon show the outlines of the two anther-lobes united by the con- 

 nective; the filament is still very short, subsequently it also grows slowly, and it 

 is only just before the expansion of the flower that it elongates very rapidly by 

 vigorous intercalary growth. When the four pollen-sacs make their appearance 

 externally on the young anthers as longitudinal protuberances, a layer of cells 

 becomes differentiated within them from the hitherto homogeneous tissue. The 

 young anther consists at first of a small-celled proto-meristem in which a fibro- 

 vascular bundle becomes differentiated lying in the axis of the connective; the 

 external peripheral layer forms the dermatogen or the young epidermis. According 

 to Warming's comprehensive researches, it is usually only that layer which imme- 

 diately underlies the epidermis (the most external layer of the periblem) which gives 

 rise to the mother-cells of the pollen and to layers of the wall of the pollen-sac 

 surrounding them. [These cells constitute the archesporium of Goebel. It will 

 be seen that in the pollen-sac, as in the sporangia of the Vascular Cryptogams, 

 the archesporium is derived from hypodermal cells.] Within each of the before- 

 mentioned four longitudinal protuberances, the layer of cells (archesporium) imme- 

 diately underlying the epidermis splits into two, the inner of which gives rise to 

 the mother-cells of the pollen. These cells soon become conspicuous on account 

 of their large size, and, on a transverse section of the anther, they are seen to 

 form a multicellular layer, concave internally, in each of the four protuberances. 

 It does occur that the transverse section of one of these protuberances shows 

 only a single mother-cell, so that here one row only of these cells is present 

 (Compositge, Malvaceae). Less frequently the mother-cells are isolated (as in 

 Mimosese). The mother-cells of the pollen divide but rarely until the formation 

 of the tetrads begins; hence the number of mother-cells primarily formed is only 

 slightly increased: but the simple row or layer of primary mother-cells may 

 form several layers or a cylindrical mass of mother-cells as the result of divisions 

 in all directions. That layer of cells which, as mentioned above, was formed by 

 the longitudinal splitting of the layer (archesporium) from which the mother-cells are 

 derived, lies between them and the epidermis. It divides, according to Warming, 

 into usually three layers, in which radial, horizontal, and tangential divisions succes- 

 sively occur. The innermost of these three layers (Fig. 374, A ep, Fig. 377, Bn), 

 which is completed by a corresponding layer on the inner side of the group of 

 mother-cells, developes into a peculiar epithelium (the tapetum), investing the 

 mother- cells of the pollen on all sides and lining the cavity of the pollen-sac ; it 

 corresponds to the tapetum in the sporangia of the Vascular Cryptogams, and, 

 like it, it is finally absorbed in the process of the development of the pollen. 

 The same fate overtakes the next outer layer of cells. The most external of 

 the three above-mentioned layers, which lies immediately beneath the epidermis. 



