DEVELOPMENT OF PLANTS 



351 



scopic, curving away from each other when the sporangia are 

 ripe, thus permitting the opening of the sporangia and the grad- 

 ual distribution of the spores by the wind and closing when mois- 

 tened to protect the spores against wetting. In some of these gen- 

 era the outer coat of the microspore is Hfted away from the 

 inner coat in such a way as to form two sac-Hke outgrowths 

 which render them more buoyant and adapted to distribution by 

 the wund (Fig. 254). The microspores are produced in such 

 enormous numbers that the ground in the vicinity of the ever- 

 green forests is often covered with the yellow spores, the so- 

 called "sulphur snow." This extravagant formation of micro- 

 spores is a necessity owing to the small chance of any one of the 

 microspores being carried to the megasporophylls and so ensur- 

 ing fertilization. Were it not for the fact that the microspores 

 need be carried only comparatively short distances, this order 



Fig. 252. Strobili of megasporophylls: i, a branch that has developed 

 in the spring from a bud. The fascicles of leaves are just emerging from 

 their papery bracts and at the right is shown a strobilus, st, consisting of 

 minute spine-tipped scales. 2, growth of strobilus shown in i, twelve months 

 later, j, appearance of strobilus shown in 2, six months later. — H. O. Hanson. 



