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. sylvaticuvi are 

 shown in figs. 190 ^ 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=^). 



Horse-tails. 



i Summer Shoot of Equisetum arvense. 2 Vernal fertile Shoot of Equisetum. arvense. s Spike of whorled sporangiophores 

 from the same Equisetum. * A single sporangiophore. ^, 6 Spores. ? Equisetum sylvaticum. s Prothallium of a Horse- 

 tall. ', -', 7 natural size ; 3x3; * 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 

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

 becomes a spore. 



