330 GENERAL MORPHOLOGY OF PLANTS 



498. The Christmas fern (Aspidium acrostichoides = 

 Polystichum acrostichoides). This is one of the shield ferns. 

 It grows in shady woods. The stem is very short and upright. 

 The leaves are pinnate, and are of two kinds, sterile and fertile. 

 The outer ones are often sterile, while the inner ones in a cluster 

 are often fertile. These are easily distinguished from the sterile 

 ones, because the pinnae which bear the spore cases are very 

 much shorter and are narrower than the sterile ones, even on the 

 fertile leaves. It is only the upper third, or half, of the leaf, 

 which bears fertile pinnae. The sori form two crowded rows on 

 the underside of each pinna. Each sorus is covered by a shield- 

 shaped structure called an indusium, which protects the spore 

 cases when they are young, but dries and withers as they ripen, 

 so that the spore cases dry out, open, and scatter their spores. 



499. The bracken fern or brake (Pteris aquilina = Pteri- 

 ^ium aquilinum). This is one of the large, coarse ferns which 

 grow in open sunny places, sometimes covering large areas and 

 becoming a nuisance as a weed. The stem is a hard, black, 

 somewhat woody structure, and grows as a root-stock several 

 centimeters (8-12 cm. = 3-5 inches) under the surface of the 

 ground. The leaves are the only portion of the plant seen above 

 the ground. They have long, stout, shining, blackish stalks or 

 petioles. The leaf is first divided in a palmate manner into 

 three stout branches (ternately divided) and each of these 

 branches is bipinnate, the final pinnae forming narrow, thin 

 lobes. The spore cases (sporangia) form a long sorus near the 

 margins of the underside of the pinnae, and the margin is incurved 

 over them for protection. 



500. Structure of the spore cases (or sporangia) of ferns. 

 In most of our common ferns (in the family Polypodiacex] the 

 structure of the spore case is as follows. There is a slender stalk 

 consisting of about three rows of cells. This supports the spore 

 case. This is a rounded, somewhat compressed or biconvex 

 structure, with a wall of a single layer of cells. As seen from a 

 side view there is a row of specialized cells which extends from 

 the stalk upward and over the top about three-fourths the dis- 



