PTERIDOPHYTES 



305 



Pericycle 

 \ Cortex 



SUMMARY 



As regards their vegetative structures the ferns may be character- 

 ized as follows : 



1. They are the simplest vascular plants and have become adapted 

 to a moist mesophy tic environment by the formation of large, thin, 

 dark-green leaves. 



2. They are really herba- 

 ceous perennials, with a sea- 

 sonal life quite comparable 

 to that of many seed plants 

 with a perennial underground 

 stem. The tree ferns of the 

 present day and those of the 

 coal period are exceptions to 

 this general herbaceous habit. 



3. A few ferns are adapted 

 to dry conditions, and some 

 are hydrophytes. 



4. The anatomy of the 

 fern rhizome, or stem, has 

 the same general tissue ar- 

 rangements as the herbaceous 

 stems of the higher flower- 

 ing plants, with a large pith, 

 a wide cortex, and a narrow 

 wood ring. The distinctive 

 features are the conspicuous 

 leaf gaps (which often trans- 



Duct Parenchyma [ Endodermis 

 Parenchyma Sieve tube 



FIG. 174. Vascular tissues of the rhizome 

 of Pteris in long section 



After Jeffrey. Redrawn from Jeffrey's 

 " Anatomy of Woody Plants " 



form the vascular cylinder into a meshwork instead of an unbroken 

 cylinder) and the concentric arrangement of phloem and xylem. 

 There is no cambium layer and consequently no increase in diam- 

 eter by the production of annual rings of phloem and xylem. The 

 water-conducting elements are more primitive in type than those of 

 the higher seed plants studied earlier in the course. 



ASEXUAL REPRODUCTION 



The leaves of ferns bear the asexual spores in definite organs, 

 called sporangia, located on the lower side of the leaves along 

 the veins. These sporangia are borne in groups called sori 



