SECTION 17.] 



PTERIDOTHYTES. 



159 



surface of the leaf or frond, or over the whole surface when there are no 

 proper leaf-blades to the frond, but all is reduced to stalks. Commonly the 

 spore-cases occupy only detached spots or lines, each of which is called a 

 Sorus, or in English merely a Fruit-dot. In many Ferns these fruit-dots 

 are naked ; in others they are produced under a scale-like bit of membrane, 

 called an Indusium:. In Maidenhair-Ferns a little lobe of the leaf is folded 

 back over each fruit-dot, to serve as its shield or indusium. In the true 

 Brake or Bracken (Pteris) the whole edge of the fruit-bearing part of the 

 leaf is folded back over it like a hem. 



488. The form and structure of the spore-cases can be made out with 

 a common hand magnifying glass. The commonest kind (shown in Fig. 

 503) iias a stalk formed of a row of jointed cells, and is itself composed 

 of a layer of thin-walled cells, but is incompletely surrounded by a border of 

 thicker-walled cells, forming the Ring. This extends from the stalk up 

 one side of the spore-case, round its summit, descends on the other side, 

 but there gradually vanishes. In ripening and drying the shrinking of the 

 cells of the ring on the outer side causes it to straighten ; in doing so it 

 tears the spore-case open on the weaker side and 

 discharges the minute spores that fill it, com- 

 monly with a jerk which scatters them to the 

 wind. Another kind of spore-case (Fig. 507) 



is stalkless, and has its 



ring-cells forming a kind 



of cap at the top : at ma- 

 turity it splits from top 



to bottom by a regular 



dehiscence. A third kind 



is of firm texture and 



opens across into two 



valves, like a clam-shell 



(Fig. 50S a ) : this kind 



makes an approach to the 



next family. 



489. The spores germi- 

 nate on moistened ground. 



In a conservatory they 



may be found germinating 

 on a damp wall or on the edges of a well-watered flower-pot. Instead of 

 directly forming a fern-plantlet, the spore grows first into a body which 



Fig. 509. A young prothallus of a Maiden-hair, moderately enlarged, and an 

 older one with the first fern-leaf developed from near the notch. 510. Middle por- 

 tion of the young one, much magnified, showing below, partly among the rootlets, 

 the antheridia or fertilizing organs, and above, near the notch, three pistillidia 

 to be fertilized. 



