HORSETAILS AND LYCOPODS. HETEROSPORY 249 



the same. Instead of large compound fronds they 

 have small scale-like leaves, and in the Horsetails 

 these are so small that the great bulk of the photo- 

 synthesis is done by the surface tissues of the stem. 

 Another point in which they differ from the Ferns is 

 that their sporangia are borne in cones, i.e. in con- 

 nexion with small crowded leaves, mostly differing more 

 or less from foliage leaves, at the ends of some of the 

 shoots (Fig. 39, A). The prothalli also are unlike those 

 of Ferns, being cylindrical or thin, branched structures 

 instead of flat. In many of the Lycopods the pro- 

 thalli are not green, but live saprophytically in humus 

 or humous soil. These prothalli grow very slowly, 

 and the sporophytes depend very largely on vegetative 

 reproduction. The British species of Lycopods live 

 mostly on the hills of the north and west where the 

 soil is humous, i.e. contains a large amount of plant 

 debris, which decays very slowly indeed, owing to the 

 cool damp climate. The Horsetails mostly live in 

 marshes, by pond sides and in damp places generally, 

 but one species, the Field Horsetail (Equisetum arvense), 

 grows very commonly on hedgebanks and the edges of 

 fields, though on stiff, moisture-retaining soils. It is 

 the only British Pteridophyte occurring in such situ- 

 ations and holding its own with the dominant seed 

 plants the common hedgerow plants. Like the other 

 Horsetails, it has long underground stems which spread 

 below the soil and send up the green aerial shoots at 

 intervals. 



Heterospory. In the great majority of existing 

 Pteridophytes the spores are all alike, but in a 

 few two kinds of spores are produced, large mega- 

 spores, much larger than ordinary spores, only one or 

 four being produced from a single sporangium, and 



