HORSETAILS 197 



contract and wrap themselves round the spore 

 when damp, but straighten out when the air is 

 dry; they thus set the spores in motion. This 

 mechanism has something to do with the dis- 

 persal of the spores, but botanists have not quite 

 made up their minds how it acts. 



The spores, as has been said, are all of one 

 kind. Yet the prothalli are almost always of 

 separate sexes, the male individual being much 

 smaller than the female. The difference, however, 

 does not depend on the spores, but on the condi- 

 tions under which they grow. If they germinate 

 on a barren soil, such as wet sand, they grow into 

 male prothalli, but if properly supplied with 

 food-materials, a female prothallus is the result. 

 The prothalli are green, and, on the whole, fairly 

 similar to those of the Ferns, as are also the 

 sexual organs of reproduction. The spermato- 

 zoids have numerous cilia, just as in Ferns. It 

 is rather interesting that the sexual generation 

 (prothallus) should be so much alike in the two 

 groups, while the asexual plant is so totally 

 different. 



The little group of twenty species or so form- 

 ing the genus Equisetum represents the last stage 

 in the history of an ancient family, dating far 

 back into Palaeozoic times. We have a very fair 

 knowledge of their history, and the further we 



