LIFE HISTORIES 21 



takes place, and even an embryo or young 

 plant may be formed. 



There remains to be considered the life history 

 of the Horsetails. The spores are always pro- 

 duced on special processes, which are arranged 

 in the form of a cone at the apex of the stem. 

 The sporangia are borne on curious scales which 

 are supported by stalks placed in the centre. 

 These scales are arranged in whorls round the 

 centre of the stem, and there may be twenty 

 or more in each row. On the underside of these 

 scales we shall find the sporangia almost any 

 number of them up to ten. Each sporangium 

 produces a considerable number of spores, so 

 that every cone is responsible for an enormous 

 number. These spores are all of one kind, and 

 they are so singular that they are worth a 

 somewhat detailed description. The covering 

 of the spore really consists of four layers, the 

 outermost of which is split spirally in such a 

 way that two long arms with flattened ends are 

 produced. As long as the spore is damp these 

 remain closely gathered round, but under dry 

 conditions they are stretched out. The move- 

 ments of these arms or elaters, as they are 

 called, are readily watched under a microscope. 

 By gently breathing on the spores we bring 

 them under the influence of moist air, and this 

 causes the elaters to curl up ; after a while, 

 however, when they become dry, the arms 

 stretch out again. It is not certainly known 

 what is the use of these elaters. A very little 

 observation shows that the opening and closing 

 of the arms keeps the spores on the move ; 

 this would aid dispersal at the time of the 

 bursting of the sporangium. Another point 

 which is worth consideration is that although 

 the spores appear to be exactly the same, yet 



