THE MICROSPORANG1UM 41 



this standpoint, there is no objection to speaking of the 

 four groups of mother-cells in Lemna as four sporangia, which 

 have had quite an exceptional origin. The phenomenon is 

 too unique as yet among Angiosperms to justify any generali- 

 zation. 



The growth of mother-cells and the enlargement of the spo- 

 rangial cavities usually result in reducing to a thin plate the 

 sterile tissue separating the two sporangial cavities on each side 

 of the anther. As dehiscence approaches, this plate usually 

 disappears, and the two sporangial cavities become fused into 

 a single loculus of the anther (Fig. 7). In the mature condi- 

 tion, therefore, such an anther contains two loculi or " pollen- 

 sacs." While this represents the ordinary condition of the 

 mature anther, among the Araceae it is reported that the single 

 loculus of the anther is formed by the fusion of four sporan- 

 gial cavities, and in Sassafras it is well known that the four 

 remain separate. In case an anther contains only two sporan- 

 gia, as among Asclepiadaceae, there is no fusion, and each 

 loculus is a single sporangial cavity. 



The mechanism for the dehiscence of anthers is extremely 

 varied (Fig. 15), and needs much more investigation than it 

 has received. By far the most common method is by means 

 of a longitudinal fissure, a definite stomium developing, as in 

 L ilium (Fig. 7), and opening by means of the drying of the 

 anther-walls, the contraction of the epidermal cells being 

 greater than that of the endothecial cells with their thick bands. 

 There is also dehiscence by a short apical fissure, as in Solanum 

 and certain Ericaceae ; by a terminal pore, formed by the dis- 

 organization of a small portion of tissue, as in certain Erica- 

 ceae ; by hinged valves, as in Berberis, Sassafras, and Hama- 

 melis ; and by irregular breaking and exfoliating of superficial 

 tissues, as in the axial stamens of Naias. The details of these 

 methods, and of others, should be investigated from the stand- 

 point of the development of the mechanism, for such as we have 

 are too vague and superficial to be of much significance. 



The mother-cell stage of the microsporangium is regarded 

 as the end of the history of the sporophyte in this direction, 

 chiefly because the division of the mother-cell, preceded by a 

 more or less prolonged rest, is a reduction division, and in con- 

 sequence the resulting cells have the feature most characteristic 



