GROUP IV. — PHANEROGAMIA : ANGIOSPERM^. 



519 



by suppression (e.g. carpellary flowers of Lanrus nohilis), as well as 

 in others where suppression of the pollen-sacs is incomplete {e.g, 

 Ganna, TroUius, some Lauraceae) ; in the latter case the stamin- 

 odia are frequently petaloid. In many acyclic flowers {e.g. 

 Nymphaea), the stamens and the petals are connected by inter- 

 mediate structures, of which it is difficult to say whether 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 (Lauraceae) 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 — (bilo- 

 cular anther) : this may be due to 

 the non-development of one longi- 

 tudinal half of the anther (e.g. 

 Cucurbitacepe, Salvia, Canna) ; or 

 to branching (e.g. Adoxa, Malva- 

 ceae) ; or to the abortion of one 

 pollen-sac, generally the posterior 

 one, of each pair (e.g. Asclepia- 

 daceae, Grubbia) ; or (some Laura- 

 ceae) of the upper or lower one of 

 each pair; or, finally, to the early 

 fusion of the archesporia of two 

 adjacent pollen-sacs (some Orchi- 

 daceae). In the Araceae the process of fusion is carried to such 

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

 locular. 



Rarely (e.g. Sarcophyte, among Balanophoracese) the anther 

 bears numerous pollen-sacs : the pollen-sac is sometimes multi- 

 locular (see p. 433). 



Each pollen-sac encloses an ai*chesporiam 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. 281 t), which eventually be- 



FiG. 333.— Part of a staminal flower 

 of Ricinus communis cut through length- 

 ways : / / the basal portions of the 

 compoundly-branched stamens ; a the 

 anthers. (After Sachs.) 



