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Elementary Biology. 



each of which subdivides into fcur daughter spore -cells, 

 which in turn become, after the development of exo- and 

 endo-sporium, the spermospores. As in the case of the 

 spores of the fern, the exosporium (or extin) is ridged or 

 elevated into spines, or otherwise variously sculptured. 

 There are numerous apertures in the exosporium through 

 which the endosporiurn (or intin) may be seen. Careful 

 examination of the spermospore when fully developed reveals 

 a structure highly suggestive of that of the spermospore of 

 Selagintlla, namely, the division of its contents into two por- 

 tions, a vegetative cell or cells and a reproductive cell. 

 The reproductive cell, however, does not undergo further 

 division, and forms no sperms. Both cells are nucleated, 

 and the much smaller vegetative cell is separated off from 

 the other by a cellulose cell-wall. The ultimate fate of 

 these cells we shall see later on, after we have studied the 

 ovosporangium and its contents. 



The development of the carpel. The carpel, like the 

 stamen, originates as a prominence of the thalamus, and 

 after developing into a leaf-like structure forms on its in- 

 turned edges minute projections the future cvosporangia 



FIG. 86. DEVELOPMENT OF THE OVULE. (Luerssen.) 



For explanation see text. 



(fig. 86, A, B). The projection becomes multicellular, and 

 is covered over by the epidermis of the placenta. The pro- 

 jection gradually increases in size until it becomes differen- 

 tiated into a head and stalk. The head forms the imcellu* 



