378 EQUISETALES. 
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 8). 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 Z. dimosum. As the sporogenous group 
enlarges a layer of cells immediately adjoining it externally becomes glandular 
Fic. 206. 
* 
Equisetum arvense, L. A, radial longitudinal section of part of young strobilus, 
showing two sporangiophores in a very young state. B,C, D, 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. 
