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 2 and 190 7 . 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 (cf. fig. 190 3 ). 



Fig. 190. Horse-tails. 



1 Summer Shoot of Equisetum irvense. 2 Vernal fertile Shoot of Equisetum arvense. Spike of whorled sporangiophores 

 from the same Equisetum. * A single sporangiophore. , Spores. T Equisetum sylvaticum. Prothallium of a Horse- 

 tail, i, 2, ^ natural size ; x 3; * x 6 ; 5 , 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 (cf. fig. 190 4 ). 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. 



