FERNS, FERN ALLIES, AND FOSSILS. 279 



prefer the bare and barren soil of railway banks and 

 shale heaps ; others, such as E. liinosuin and E. palus- 

 tre^ live either in water or in very moist places. They 

 do not seem able to hold their own on good soil, if 

 other plants are allowed to extend over it. 



The " flower " consists of very numerous spore-leaves 

 which are peculiar in shape, and are on a much higher 

 scale of specialism than those of the ordinary ferns and 

 Clubmosses. The spore leaf has a short stalk and ends 

 in a flat brown hexagonal or pentagonal top; when young 

 these flat top-pieces fit together, forming an absolutely 

 continuous surface over the stem ; the sporangia, which 

 are hung on the undersides of these tops, are thus 

 thoroughly protected against rain, or other dangers. 

 Very often the whole of the young flower is, in addition, 

 covered by several series of leaf whorls, which become 

 distant, and separate when the stem elongates. 



The spores are scattered very early in spring ; the 

 Equisetum " flower " is one of the very first to appear ; 

 when ripe, the spore leaves become slightly separated 

 through the growth of the axis to which they are 

 attached, and the spores are carried off by the wind. 

 They have a very peculiar arrangement for distribution, 

 an arrangement which is apparently entirely confined to 

 this one genus. The spores have three membranes or 

 cell walls. The outermost of these membranes or coats 

 becomes divided along a line which runs spirally round 

 the spore, and forms four ribbons, which are only 

 attached to the spore at one spot. These ribbons are 

 very hygroscopic, and they curl up, or uncurl and twist 

 about, according to the amount of moisture in the air, 

 so that the spore shifts its position, and has a better 

 chance of reaching a favourable place for germination. 

 The spores form a prothallium upon which antheridia 



