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, in 

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



Fig. 190.— Horae-tails. 



' Summer Shoot of Equisetv/m arvense. 2 Vernal fertile Shoot of Equisetum arvense. s Spike of wnorled sporangiophores 

 from the same Equisetum. * A single sporangiophore. fi, « Spores. 7 Equisetum sylvaticuTn. 8 Prothallium of a Horse- 

 taU. 1, 2, ' natural size ; « x 8; * x 6 ; «, « x 26 ; » 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 ceU-layers 

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

 tissue breaks up into cells. These cells then divide into four cells, each of which 

 becomes a spore. 



