THE ANGIOSPERMAE 1199 



The middle layer is sometimes confined to the walls of the sporangial 

 lobes and sometimes it surrounds the whole anther. It does not, however, 

 as a rule appear, at least in its characteristic form, on the inner side of the 

 lobes, between them and the tissue of the connective. Although the anther 

 wall in the majority of families has only one thickened middle layer, cer- 

 tain exceptions may show from two to five or more, the highest numbers of 

 layers being in Agave and Iris. 



Between the thickened layer and the archesporium there may be only 

 one layer of cells or there may be several. The innermost layer always 

 develops into the tapetum and if there are any supernumerary layers they 

 are generally temporary and are sooner or later flattened and crushed. The 

 tapetum surrounds the archesporium. Its cells divide freely and form a 

 layer of enlarged cubical cells, which elongate centripetally as the arche- 

 sporium becomes transformed into pollen mother cells. Their protoplasm 

 becomes dense and pigmented, usually yellow or orange, but occasionally 

 pink [Ktiaiitta), reddish-brown {Pyrus) or violet {Anemone). As the tapetum 

 completely surrounds the archesporium, it follows that one sector of it 

 originates from the sporangial wall and another sector from the tissue of the 

 connective, but it is nevertheless uniform all round. 



There are two kinds of tapetum. One is known as the Secretory Type, 

 found only in the Lycopodiales and Isoetales, in which the cells do not 

 disintegrate, and the other, the Plasmodial Type, characteristic of the 

 Filicales, Equisetales and most Spermatophyta, in which the cell walls 

 dissolve, the nuclei multiply and become distorted in shape and the liberated 

 protoplasm, which now forms a united plasmodium, sends out long pseudo- 

 podia which penetrate between and envelop the pollen mother cells. This 

 reversion to a state of free protoplasmic movement in highly organized 

 plants is a very striking phenomenon. The tapetum disappears when the 

 pollen is ripe and before the anthers open. (See Volume I, Fig. 546.) 



As the pollen matures the part of the sporangial wall separating the two 

 neighbouring lobes, on each side of the anther, is more or less completely 

 broken down, the two pollen-sacs becoming united into one. It is in this 

 state that the anther is described by systematists as " bilocular ". The two 

 sporangial walls, however, remain conjoined by the tissue which lies at the 

 bottom of the furrow between them. Here the cells of the epidermis are 

 much smaller than elsewhere, forming a strip known as the stomium. 

 Eventually they break down or disjoin, opening a longitudinal slit between 

 the sporangia, which constitutes the line of dehiscence. This may be 

 simultaneous all down the length of the anther or it may be progressive 

 from apex to base. 



These various stages may all be passed through while the flow^er is still 

 a closed bud. When it opens and air and sunlight strike on the delicate 

 anther tissue, the sporangial walls begin to lose water and to contract. 

 This is due to the hydrostatic tension developed and the cohesion of the 

 contained water to the cell walls. (See also under Nectaries, p. 125 1.) In 

 the important middle layer this contraction affects the unthickened outer 

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