16 STRUCTURAL BOTANY 



III. REPRODUCTION AND LIFE-HISTORY 



a. The Sporangia and Spores 



We have already learnt that Selaginella is repro- 

 duced by spores, which are of two kinds. The organ 

 in which the spores are immediately produced is the 

 sporangium. In Selaginella a single sporangium is 

 borne in the axil of each fertile leaf or sporophyll of the 

 cone. The cone is a vertical shoot differing but little 

 from the ordinary vegetative shoots of the plant, and 

 bearing many sporophylls (see Figs. 3, 4, and 5). 



We will now trace the development of a single 

 sporangium. It does not matter whether we take a 

 microsporangium or a megasporangium, for up to a 

 certain point they develop in the same way. 



Each sporangium arises just below the growing-point 

 of the cone, from the outgrowth of a little group of 

 meristematic cells, situated either exactly in the axil 

 between leaf and stem or rather higher up, on the stem 

 itself. A little ridge of tissue is thus produced, which 

 at first consists of uniform cells. Very soon, however, a 

 few cells in the middle of the young sporangium, lying 

 immediately below its epidermis, begin to be distinguished 

 by their more abundant protoplasm (see Fig. 9, B). Only 

 one or two such cells are visible in a radial section such 

 as that shown in the figure. This little group of cells is 

 called the archesporium (see Part I. pp. 113, 264), for 

 ultimately, after much growth and numerous cell divisions, 

 it produces the spores. The archesporium soon becomes 

 surrounded by a well-marked layer of cells, the tapetum, 

 formed partly from the surrounding tissue, and partly 

 from the archesporium itself (see Fig. 9, A and B t t). 



