THE VASCULAR CRYPTOGAMS 99 



then breathing on them. The moist air makes the elaters 

 coil up, and as they dry they stretch out again, setting 

 the spores in motion by their contraction and expansion. 



The use of this curious arrangement has been a good 

 deal discussed. Probably the chief function of the 

 elaters is to help in the dehiscence of the sporangium. 

 As this loses moisture the spores inside begin to stretch 

 out their elaters ; this causes the whole mass of spores 

 to take up more room, and so to press on the wall of 

 the sporangium, which they thus tend to burst. Another 

 use of the elaters may be that they cause the spores 

 to keep entangled together, so that they are obliged to 

 germinate in company. This may be of importance, as 

 the prothalli are usually dioecious. 



The outermost layer of the sporangial wall, which alone 

 persists till maturity, consists of spirally thickened cells. 

 Dehiscence takes place by a longitudinal slit (Fig. 38, 4). 



The development of the cones, at least in some species 

 of Equisetum, is remarkably slow. Thus the cone from 

 which the section shown in Fig. 44 was made, would not 

 have matured for two years. In this species (E. 

 maximum, the largest in the British Flora) the cones of 

 three successive years are present on the plant at the 

 same time. In March, when the spores are shed, we 

 have not only the ripe cones rising into the air on the 

 fertile stems, but underground, still enclosed in buds, we 

 find the cones of the next year, and the year after that 

 too. In some species, however, as in E. limosum, the 

 development is a great deal quicker. 



The spores of Equisetum contain not only a nucleus, 

 but chlorophyll-granules. As is usually the case with 

 green spores, they must be sown within a few days after 

 ripening, or they will not come up at all. 



