THE STAMEN 141 



veloped fibrous layer. Genera of the Urticaceae, with a uniseriate wall 

 and no fibrous layer, show the extreme reduction structure. Transition 

 forms occur in some Moraceae, the Ulmaceae, and Cannabinaceae. In 

 Dorstenia and Broussonetia (Fig. 59B, C), a few epidermal cells are 

 present over the region of wall fusion between the two sporangia of 

 the anther sac. Hiimuhis (Fig. 59A), Ulmus, and Cannabis show epi- 

 dermal cells scattered over the fibrous layer. The isolated epidermal 

 cells (Fig. 59G, H) are survivors of the typical epidermis of earlier 

 stages of anther development. The simple wall of the Urticiflorae is the 

 result of reduction. Evidence is the presence of isolated epidermal 

 cells, survivors of a typical epidermis of earlier ontogenetic stages, and 

 the absence of the layer of wall cells below the fibrous layer, also often 

 suppressed in ontogeny. 



Explosive dehiscence characterizes many of the genera of the Urtici- 

 florae, especially those of the Urticaceae. The pollen grains are hurled 

 into the air by sudden bursting of the anther-sac wall. A remarkable 

 feature of this type of dehiscence is the simultaneous explosion in some 

 tropical genera of great numbers of anther sacs, forming smokelike puffs 

 of pollen grains. The mechanism of explosive dehiscence is not under- 

 stood. It has been reported to be the result of uniform shrinkage of a 

 uniseriate wall, of internal turgor, and of pressure of maturing pollen 

 grains upon a delicate enclosing wall. A structural feature of the anther 

 sac, unknown in other types, may be a part of the mechanism: there is 

 a short, longitudinal slit in the wall of the young anther. Explosive 

 dehiscence is a highly specialized type developed in association with 

 anemophily. 



In Casuarina, the simple anther wall suggests superficially the spo- 

 rangium wall of gymnosperms (Fig. 59G), but its outer wall is a 

 typical fibrous layer and the epidermis persists, as a few isolated cells, 

 over the partitions between the sporangia. The anther is not explosive 

 but dehisces normally. In the Proteaceae, the anthers of some taxa — 

 Leucadendron — have lost the epidermis completely, and others — species 

 of GreviUea — have only scattered surviving cells. In the Piperaceae, 

 Pepewmia also has persisting remnants of the epidermis. 



In submersed aquatic plants, the entire plant is structurally reduced 

 in adaptation to its habitat. The anther sacs of these plants — with the 

 exception of Zostera and Zannichellia — have uniseriate walls. Zostera, a 

 transitional form, has a biseriate anther wall. The outer wall layer is of 

 large parenchyma cells — a true epidermis; the inner layer is of thin 

 cells with weak wall thickenings, which do not function in dehiscence. 

 The pollen is freed by local swellings that break open the sac. 

 Zannichellia has a simple, biseriate wall (Fig. 59D). The swelling of a 

 small cluster of cells over the septum ruptures the wall (Fig. 59E). In 



