FERNS 



359 



from the spores, is thallose, and called a prothallus or prothallium. 

 It is quite small, thin and heart-shaped in many species, and 

 bears the sexual organs on the underside. There are also rhi- 

 zoids which attach it to the substratum and supply water and 

 mineral foods, while chlorophyll in the prothallium enables it to 

 make its own carbohydrate food. It is thus able to lead an 

 independent existence. The character of ferns can be observed 

 by the study of a few examples. 



SOME OF THE COMMON FERNS. 



512. The polypody fern (Polypodium vulgare). This is 

 one of the common ferns. It grows in the open woods, often 

 near cliffs, on the ground or on rocks. The stem is creeping (a 



Fig. 319. 



Christmas fern (Aspidium 

 acrostichoides). 



Fig. 320. 

 Rhizome of sensitive fern (Onoclea sensibilis). 



root-stock). On the ground the stem often lies just beneath the 

 leaves, while on the rocks it is usually exposed. Near the grow- 

 ing end it is covered with numerous brown scales. The roots 

 are numerous, finely fibrous and black. The leaves are very 

 conspicuous and arise in a cluster from the apex of the stem. 

 They are once pinnate, the blade being divided to the midrib 

 on either side into numerous linear divisions. Some of the 

 leaves are sterile while others are fertile. In the fertile ones 

 the spore cases are collected into roundish groups (sori, a single 

 one a sorus), these groups forming two rows, one on either side of 

 the mid- vein of each leaflet (pinna). 



