78 



EOUISETALES 



products thus formed share with the product of the inner cell already 

 described in constituting the large sporogenous tissue, which, though entirely 

 derived from the single parent cell, is not defined by its first periclinal 

 wall : it is indicated by shading in the figures, while the products of the 

 subsequent periclinal divisions are marked with a cross. Transverse sections 

 at the stage represented in Fig. 207 A show the sporogenous tissue in a 

 central position surrounded by several rather irregular layers forming the 

 sporangial wall (Fig. 207 B). The size and construction of the sporangia, 

 even of those in near juxtaposition, may vary greatly : this has been 

 especially seen in the case of E. limosum. As the sporogenous group 

 enlarges a layer of cells immediately adjoining it externally becomes glandular 



FIG. 206. 



Equisetum arvense, L. A, radial longitudinal section of part of young strobilus, 

 showing two sporangiophores in a very young state. B, C, >, individual sporangia, in 

 older states, cut in median section. X 200. 



in appearance, and develops as the tapetum (Fig. 208 A). Later the cells 

 of the sporogenous tissue itself separate, and round themselves off as spore- 

 mother-cells ; but it is only about two-thirds of these cells which undergo 

 the tetrad-division, about one-third of them shrivel, and become disorganised, 

 their substance mingling with that of the tapetum, which becomes intrusive 

 as a multinucleate plasma into the interstices between the spore-mother- 

 cells (Fig. 208 B) : the fertile cells which remain are nourished by this 

 as they develop into the mature spores. Finally the superficial cells of 

 the wall become indurated and spirally thickened, while those within it, 

 excepting at the base of the sporangium, are disorganised. The mature 

 sporangium, consisting thus of a single layer of cells of the wall, and 

 containing the ripe spores which are all alike, dehisces along a longitudinal 

 line facing inwards towards the stalk, which line had previously been defined 

 by the cell-structure. 



