14 SPORES AND THALLIDIA. 



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

 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 (cf. fig. 190^). 



Fig. 190.— Hor 



* Summer Shoot of Equisetum arvense. 2 Vernal fertile Shoot of Equisetum arvense. 8 Spike of whorled sporangiophorea 

 from the same Equisetum. * A single sporaDgiophore. ', « Spores. ' Equisetum sylvaticum. 8 Prothallium of a Horse- 

 tail ',',' natural size ; » X 3; < 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 {cf. 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. 



