24 THE LIFE-HISTORY OF A FERN 
In this connection it is well to note further that the spores are produced. 
upon the leaves fully exposed to the air, and that dry circumstances 
favour the shedding of the spores: Ferns grown in uniformly moist con- 
ditions show- how essential a dry period actually is, for their sporangia 
often do not burst at all. The spores of Zodea and of some Hymeno- 
phyllaceae may even be seen germinating within the sporangium. Such 
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Fic. 8. 
Dispersion of the spores from sporangium of Aspidium acrostichoides, showing different 
stages of the eversion and snapping of the annulus. (After Atkinson.) 
a condition is obviously of no advantage to the plant, and is to be 
looked upon as a failure in the normal action of the annulus. We thus 
see that a relatively dry period, such as the Male Fern is able to undergo 
in summer,-ts a normal state, and indeed essential for the last phase of spore- 
production, viz. the dissemination of the numerous living germs. 
But the relatively dry conditions which lead up to and are necessary 
for the dissemination of the spores do not suffice for their further 
development: in order that they may germinate moisture is required, 
as it is also throughout the immediately succeeding stages of life. When 
