292 



Introduction to Botany. 



broken away the spores are still kept from falling out by 

 means of a fringe of teeth (Fig. 150, C) ; these are more or 

 less hygroscopic in different species. 

 They close the opening of the cap- 

 sule in a humid atmosphere and 

 open outward to allow the spores to 

 be shaken out in a dry atmosphere. 

 The winds probably dry the teeth 

 and shake out the spores at the 

 same time. 



FERNS. 



179. Character of Ferns. — An 



examination of the construction of 

 a fern shows it to be much more 

 highly organized than a moss. The 

 tissues of its stem and leaves are 

 differentiated to perform separate 

 functions, as we have seen to be the 



case in the higher plants, and it has true roots. The leaves 



not only perform the usual photosynthetic function of 



green leaves, but they 



may also bear spores 



asexually on their under 



surface (Fig. 153); and 



since no part of the fern 



plant bears sperms or 



eggs, we must look upon 



it as an asexual genera- 

 tion, or sporophyte, 



which in the moss we 



saw to be leafless 



(namely, the capsule 



Fig. 153. 



Scolopendritttn, a fern bearing 

 oblong clusters of sporangia 

 {sort, singular sorus) . After 



Strasburger. 



Fig. 154. 



Cross section through a sorus of Scolopendrium. 

 a and b, sporangia containing spores (the 

 spore is the unicellular stage of the gameto- 

 phyte} produced asexually ; c, the protective 

 covering or indusium. After STRASBURGER. 



