354 



THE CEOSSINO OF FERNS. 



How TO Effect a Cross. 



It only remains to give a few hints how to ensure the 

 greatest chance of success in crossing. It is clear that the 

 closer the juxtaposition of the prothalli the greater the 

 chance of the anthorozoids from one prothallus straying on 

 to another, therefore thick sowing is a consideration ; but 

 as thick sowing is a fruitful cause of an unhealthy state of 

 the j)rothaIli (which is generally the destruction of the 

 whole), extreme care should be taken to ensure healthy con- 

 ditions in every respect. And first, perhaps, in importance 

 is it, that spores only sliould be sown: for this purj)OKo it is 

 best to have an abundance of spores, and so to manipulate 

 them as to sift them well ; with care, and the skill which 

 comes easily with a little practice, this can be done so that 

 practically only spores remain. But even more important 

 than that is it that the spores which it is intended to cross 

 with should have come to maturity about the same time ; 

 for it is well known tliat spores that have boon gathered 

 some time do not germinate so soon as those that have 

 recently dropped, and of course, unless the prothalli are fit 

 for fertilization at the same time, the whole chance of cross- 

 ing is lost. 



Other probable aids to crossing may bo a judicious water- 

 ing overhead just at the right time, and possibly it may be 

 a further help oitlier to sow on an incline, or afterwards to 

 tilt the seed-pan. I have only, in conclusion, to add, that 

 where so much is necessarily dependent on chance, one must 

 not be too sanguine ; with care, however, and porsovoranco, 

 results are certain to follow in duo course. 



