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7 HE BOOK OF 
one prothallus by severance, but as there is no doubt whatever 
that cross-fertilisation occurs through the myriad tiny insects 
that are invariably present, even then there is a risk, and in 
fact the risks, difficulties, and uncertainties are practically so 
unavoidable that a different method altogether is compulsorily 
adopted. The usual course when a cross between two distinct 
Ferns is desired is simply to collect the spores of each as 
carefully as possible with a view to eliminate strays, and then 
either to mix them intimately before sowing, which is the better 
plan, or sow them one after another rather thickly in the same 
pan. By this means a proportion of the distinct spores are 
bound to germinate in such close juxtaposition as to cause more 
or less overlapping when the prothalli develop, and obviously 
we thus produce conditions favourable to cross-fertilisation, 
especially if at the proper time, that is just before the prothalli 
are full size, we produce a small inundation, either by immersing 
the pan until the water percolates through the soil, and just 
submerges the undersides of the prothalli, or by watering 
overhead, which is less to be commended. My own idea of 
the best way is that the pan should be placed in warm water, 
say about 70 degrees or so, and when the prothalli are reached 
by the rising water to let them stand thus for half an hour 
or more, so that, stimulated by the warmth and wet, a large 
number of the ripe antheridia may burst, when their contents 
would naturally, in the prevailing flood, find their way in all 
directions, and certainly afford the maximum of chances for 
foreign alliances. Shorter immersion would reduce these, and 
a mere dip and out again would carry the bulk of the free 
antherozoids into the soil before they had a chance to wander. 
The above suggestions as to sowing together for a cross apply 
necessarily to such varieties or such species as arrive at the 
fruition stage in their prothalli about the same time. Ferns, 
however, vary greatly in this respect; some bear spores which 
germinate at once and arrive at maturity at a time when others 
have only begun to form prothalli. Obviously with such ferns 
a cross can only be arrived at either by sowing the slow one 
thinly much earlier than the other, and subsequently sowing in 
the same pan, the rapid grower broadcast among the other 
existing prothalli, or by pricking out patches from separate 
sowings which have arrived at maturity and placing these 
closely together in a pan by themselves in the hope that 
subsequent flooding or other means of transfer may lead to the 
desired results. The former course is clearly preferable if the 
cultivator is sufficiently acquainted with the relative sluggishness 
or activity to be able to calculate when to sow the second 
crop. 
