14 



SPORES AND THALLIDIA. 



Horse-tails exhibit a process of spore-formation quite peculiar to themselves. 

 Two species of this group— namely, Equisetum arvense and E. sylvaticum are 

 shown in figs. 190 ^ and 190^. At the top of the hollow stem there is a spike of 

 peltate scales borne on short stalks and arranged in whorls, each of which must, m 

 consideration of its origin, be looked upon as a metamorphosed leaf (c/. fig. 190^). 



Fig. 190.— Horse-tails. 



1 .SumnuT Shoot of Equiselum irvense. 2 Vernal fertile Shoot of Equisetum anieme. » Spike of whorled spnraiigiophores 

 from tlio saTiie Equisetum. < A single sporangiophore. ', > Spores. ' Equisetum sylmticum. ' rrotliallinin o( a Horse- 

 tail. !,■',' natural size ; « x 3; < X 6 ^ ^ « X 25 ; » x 30. 



On the inner surfaces of the scales — i.e. those turned towards the axis of the spike — 

 little warts arise, which develop into sporangia (c/. fig. 190^). The outer cell-layers 

 of these multicellular warts become the walls of the sporangia, whilst the inner 

 tis.sue breaks up into cells. These cells then diviile into four cells, each of which 

 becomes a spore. 



