EQUISETINEM. 



403 



portion of each protuberance remains slender, and forms the pedicel of the hex- 

 agonal peltate scale. The outer surface of these scales is tangential to the axis 

 of the spike ; on its inner side, facing the axis, arise the sporangia, five or ten in 

 number on each scale. In the early stages of development each single sporangium 

 has the appearance of a small blunt multicellular wart; [an axial row of the cells 

 grows more vigorously than the rest, and it is the terminal hypodermal cell of 

 this row which constitutes the archesporium. 

 The epidermal layer divides by walls parallel to 

 the surface, so that four layers of cells invest the 

 archesporium, of which the two outer form the 

 wall of the sporangium, and the two inner the 

 tapetum]. By repeated divisions the archesporial 

 cell produces the spore-mother-cells which be- 

 come isolated, while of the exterior cell-layers 

 which at first envelope them only the outermost 

 finally remains as the wall of the sporangium. 

 The mother-cells of the spores, connected to- 

 gether in groups of fours or eights, float freely 

 in a fluid which fills the sporangium and is inter- 

 spersed with granules. The processes that take 

 place in the mother-cells up to the time of the 

 formation of the spores have already been de- 

 scribed in detail in Chap. I (see Fig. 10, p. 14)^ 

 It was there shown how the division into four 

 of the mother-cells is preceded by an indication 

 of a division into two, in a manner analogous 

 to the corresponding process ili Ferns. The 

 ripe sporangium opens by a longitudinal slit 

 on the side which faces the pedicel of the peltate 

 scale. The very thin-walled cells of the wall 

 have previously formed spiral thickening ridges 

 on the dorsal, annular ones on the ventral side 

 of the sporangium, arising, according to Duval- 

 Jouve, in the case of E. hmosum, with extra- 

 ordinary rapidity immediately before the de- 

 hiscence. The development of the spores of 



Equisetum, after they have made their appearance as naked primordial cells by 

 the division into four of their mother-cells, shows the peculiarity of a successive 

 formation of distinct coats. Each spore forms first of all an outer non-cuticularised 

 coat capable of swelling, which, splitting subsequently into two spiral bands, forms 

 the so-called elalers, 3. second and third coat soon afterwards making their appear- 

 ance within it. All three lie at first closely one upon another like successive layers 

 of a single coat; but when the spore is placed in water, the outer one, even at 

 this period, swells up strongly and becomes detached from the others (Fig. 286, 



Fig. 285.— E^w'setum Te/ma/em; A upper part of 

 a fertile stem with the lower half of the spike (natural 

 size), b leaf-sheath, a the annular 'bract,':*; the pedi- 

 cels of peltate scales which have been cut off, y 

 transverse section of the rachis of the spike ; B 

 peltate scales in various positions (slightly magni- 

 fied) ; St the pedicel, s the peltate scale, sg the spo- 

 rangia. 



[On the development of the spores see Strasburger, Zellbildung und Zelltheilung, 3rd ed., p. 155,] 



D d 2 



