72 TRAXSACTIOXS, XATLRAL HISTORY SOCIETV OF GLASGOW. 



stiginatic surface in connection with the archego- 

 nium, and secures their safe delivery. Except for 

 this difference, we might regard the higher Cryp- 

 togams as wind-fertiHsed. The Gymnosperms pro- 

 bably represent transition-forms which have lost 

 their antherozoids and have not yet acquired the 

 specialised stigma of wind-fertilised flowers. The dis- 

 tribution of antherozoids from the microspores as 

 secondarj^ centres, which makes the fertilisation of 

 these Cryptogams, other things being equal, a 

 matter of much greater probability than the fer- 

 tilisation of a Gymnosperm like the pine, where 

 no special provision is made for the delivery of 

 the pollen-grain, reveals the importance of second- 

 ary centres in dispersion. When the object to be 

 gained is the delivery of a germ at a particular 

 spot, this may be attained either (first) by the 

 distribution of a vast number of such germs from 

 one centre, or (second) by the dispersion of a smaller 

 number from the primary centre, and by each of 

 the germs so dispersed itself becoming a new centre 

 of dispersion. The latter has the advantage of 

 securing a more equable distribution. This principle 

 perhaps explains a peculiarity in certain parasitic 

 fungi. The secidiospores, uredospores, telutospores, 

 and sporidia, to which Puccinia successively gives 

 rise, represent so many centres of dispersion. This 

 extreme dispersive efiiciency would appear to be 

 necessary in order that the fungus may succeed in 

 ti'ansferring itself from the Berberry to the Wheat 

 and again from the Wheat to the Berberry. Where 

 the method of secondary centres is in operation it 

 will be of advantage if, as in the higher Cryptogams, 

 bur-dock {Arctium), and spindle -tree {Euonymus), 

 an agency is employed for effecting dispersion from 

 the secondary centres different from that which 

 occasions the first sowing. The various points now 

 discussed will perhaps be better understood from an 

 illustration. The postal system of a large city 

 affords an instructive parallel. If the general post- 



I 



