340 



GENERAL MORPHOLOGY OF PLANTS 



The ostrich fern (O. stmthiopteris = Matteuccia struthiopteris) pre- 

 sents a similar dimorphism of the leaves, and also the cinnamon 

 fern (Osmunda cinnamomea) . But in the cinnamon fern the 

 spore-bearing leaves are formed during the late summer and 

 autumn. They are hidden in the crown of the leaf stalks at the 

 end of the stem during winter. In the spring they elongate and 

 unroll before the appearance of the vegetative leaves. Parts of 

 the spore-bearing leaves are often expanded into vegetative parts. 

 In the royal fern (O. regalis) the dimorphism is shown on the 

 same leaf, the tips of the divided leaves being contracted and 

 bearing spore cases, while the basal portions are expanded. In 

 Clayton's fern (O. claytoniana) the middle part of the leaf bears 

 the spore cases. 



508. Apogamy and apospory.* Some ferns have developed 

 the habit of doing away with certain stages in their life history. 



For example Pteris cretica, 

 a common fern grown in 

 greenhouses, does away 

 with the sexual organs 

 and fertilization.! The 

 fern plant (sporophyte) 

 grows directly out of the 

 tissue of the prothallium 

 (gametophyte) . This is 

 called apogamy, which 

 means without marriage. 

 Some other ferns can 

 produce prothallia di- 

 rectly from the leaves without the spores. This is apospory. It 

 is not a rare occurrence in the common brake, though the pro- 

 thallia never become more than protonemal threads in this fern. 

 The pinnae which bear these are very much contracted and wrin- 

 kled, and can be easily distinguished from the normal form of 



* For reference. 



t Recent investigations suggest that fertilization here takes place in two 

 adjacent prothallial cells by the nucleus migrating from one into the other. 



Fig. 323. 

 Apogamy in Pteris cretica. 



