GROUP V. ANGIOSPERM^E. 



463 



they are to be regarded as petaloid stamens or as staminoid 

 petals. 



The Microsporangia or Pollen-Sacs are borne on the anther. 

 There are commonly four of them (quadrilocular anther), two 

 forming a sorus in each longitudinal half (or theca) of the anther, 

 situated usually side by side, but sometimes (Lauracese) one above 

 the other ; in the former case the typical arrangement seems to be 

 that of each pair of pollen-sacs one belongs to the anterior or inner 

 surface of the anther, the other to the posterior or outer surface. 



In some cases, however, there are but two pollen-sacs (bilocular 

 anther) ; this may be due to the non-development of one longi- 

 tudinal half of the anther (e.g. Cucurbitacese, Salvia, Canna) ; or 

 to branching (e.g. Adoxa, Malvaceae) ; or to the abortion of one 

 pollen-sac, generally 



the posterior one, of ff 



each pair (e.g. As- 

 clepiadaceae) ; or 

 (some Lauracese) of 

 the upper or lower 

 one of each pair ; or, 

 finally, to the early 

 fusion of the arche- 

 sporia of two adja- 

 cent pollen-sacs 

 (some Orchidacese). 

 In the Aracese the 

 process of fusion is 

 carried to such an 

 extent that all four archesporia fuss, so that the anther is uni- 

 locular. 



Each pollen-sac encloses an archesporium from which the 

 mother-cells of the microspores (pollen-grains) are developed by 

 division ; each group of spore-mother-cells is invested by a layer of 

 granular cells, the tapetum (Fig. 279 f), which eventually becomes 

 disorganised : externally to this is the wall of the pollen-sac con- 

 sisting of one or more layers of cells with usually reticulately 

 thickened walls, followed by the epidermis at the surface. 



The pollen-sacs dehisce usually by a longitudinal slit which, 

 when the anther is quadrilocular, is generally so situated that it 

 at once opens into both the pollen-sacs of each half of the anther, 

 and frequently the tissue separating each pair of pollen-sacs be- 



FIG. 279. Transverse section of a yonng anther of Sam- 

 lucus racemosa ( x 80) : c the connective with the vascular 

 bundle ; ps the four pollen-sacs (microsporangiaj ; p the 

 mother-cells of the pollen; t tapetal layer; tc the wall of 

 the pollen-sac. 



