DISPERSAL BY WIND. 



815 



and 450 ^*) when the spores may be shaken out and blown away. As soon as the 

 sporangia are moistened the two valves immediately shut together (figs. 450 " and 

 450 '^) and obviously the spores can no longer be shaken out. A similar opening 

 and shutting of sporangia according to dryness or moisture may also be observed in 

 the Lycopodiaceae (see fig. 405'', p. 716). 



A similar phenomenon may also be observed in the sporangia of the Horse-tails 

 (see fig. 403 ■*, p. 712). Here not only the sporangia but the spores themselves present 

 a very diflerent appearance according as they are dry or damp. The wall of these 

 spores consists of several layers, of which the outermost splits up spirally to form 

 two arms which remain joined to the spore at one spot. In dry weather the two 

 spiral bands, which are arranged in the form of a cross, unroll (see fig. 451 ') and 

 constitute four appendages which afford enough purchase to the wind to enable the 

 comparatively large and heavy spores to be carried away. If the spores fall on to 



Fig. 451. — Spores of the Horse-tail K'lui.-, !i.un 'l\UnatLjd. 

 I lu dry ; - iu moist condition ; x 25. 



some spot of earth which, on account of its dryness, is not suitable for their 

 germination the wings remain widely outspread. The next gust of wind raises 

 them up again and carries them to another place. If, however, the resting-place is 

 moist, and if the conditions are favourable to the growth of the Equisetum., the 

 bands roll up spirally (see fig. 451 ^). In this way the spores may become fastened 

 to a projecting object, and if this should not be the case at least the rolling up of 

 the bands produces a diminution in size, and the spores are not easily again blown 

 away from a suitable damp resting-place. Another explanation as to the function 

 of these structures has also been ofiered, namely, that by means of the repeated 

 expansions and contractions of these hygroscopic arms the spores are linked together 

 as it were arm in arm. Further reference to this will be found at p. 713. 



The soredia of Lichens must also be mentioned as asexual reproductive bodies 

 which are distributed in enormous quantities by currents of air. To the naked eye 

 they look like a floury dust deposited in places on the Lichen thallus. These dusty 

 masses are built up of green cells, either solitary or in groups, which are wrapped 



