14 SPORES AND THALLIDIA. 



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

 Two species of this group-namely, Eqwisetum arvense and E. sylvahcum are 

 shown in figs. 190 ^ and 1901 At the top of the hollow stem there is a spike ot 

 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 (of. fig. 190 ). 



Kg. 190.— Horse-tails. 



1 Summer Shoot of Equisetmn arvense. 2 Vernal fertile Slioot of JEquisetum arvense. ' Spilie ot whorled sporangiophores 

 from tlie same Equisetwm. * A single sporangiophore. 6, 6 Spores. ? Equisetum sylvatvywm. 8 Prothallium of a Horse- 

 tail. 1, 2, ' natural size; s x 3; < X 6; 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 cell-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. 



