DEHISCENCE OF THE ANTHER 167 



dermis where the pore will develop, and the opening is enlarged by 

 shrinkage of the surrounding tissues — some of the Tremandraceae. 

 Where the anther sacs have elongate tips, the sporogenous tissue in these 

 tips may become sterile, forming delicate parenchyma cells, which dis- 

 integrate when the pore opens and leave hollow tubes extending to the 

 pore. Cellular breakdown may involve the connective tissue between the 

 tips of the anther sacs, and the two sacs — four sporangia — open through 

 one pore. Poricidal dehiscence has apparently been derived inde- 

 pendentlv in different taxa from longitudinal dehiscence by shortening 

 of the slit. It reaches high specialization where the pollen is freed from 

 four sporangia through a single pore, as in Cassia. Forms transitional to 

 longitudinal have some fibrous tissue, restricted usually to areas around 

 the pore — Solanaceae. Association of poricidal dehiscence and a fibrous 

 layer throughout the length of the sac is rare. 



Complex form in the anther is often associated with dehiscence. An- 

 thers with poricidal dehiscence may have long, tubular projections 

 ("awns") at the ends of the sacs, through which the pollen is gradually 

 sifted out. In anthers with slitlike openings, the slit may be so shaped as 

 to free a valvelike flap of tissue; this is valvular dehiscence — 

 Lauraceae, Berberidaceae. In Hamamelis, the valve flap is folded out- 

 ward and backward and carries the pollen mass out of the chamber as it 

 lifts. In some aquatic genera with submerged flowers, the pollen is freed 

 by disintegration of the anther sac. 



Although the general structure of the anther wall has already been 

 discussed, the structure and distribution of the outer layers in their 

 relation to dehiscence need further consideration. The epidermis is 

 characteristically simple but, at anther maturit)', may show unusual 

 structure. Its cells may be tabular and greatly flattened, appearing col- 

 lapsed, as in Carina, Balsamea, and many Compositae. It may be lost 

 during ontogeny, either wholly or in part — Vitis, Grevillea, Asarum, 

 many Compositae. Along the line of dehiscence, it may be absent — 

 Aristolochia, Nepenthes. A narrow band of cells may enlarge greatly 

 when dehiscence approaches — Crocus, Iris, Bignonia — and form a spe- 

 cialized opening stiucture, a stomium. The well-developed stomium of 

 Lilium is frequently figured. A crest of thickened epidermal cells may 

 develop along both sides of the line of dehiscence — Passiflora, Lycoper- 

 sicum. Epidermal hairs in a fringe along the sides of the slit perhaps aid 

 in the freeing of the pollen grains by hygroscopic movements. Thick- 

 ening bars like those of the fibrous layer have been reported in the epi- 

 dermis of Clandestina. 



The fibrous layer is obviously structurally adapted, by the uneven 

 thickenings of its cell walls (Fig. 68), as a mechanism at least largely 

 responsible for the opening of the anther sac. But there are families and 



