FERNS 



417 



pictured the vascular system forms a cylinder, but it is 

 not a cylinder with xylem on the inside and phloem on the 

 outside such as you have seen in seed plants. In this cyl- 



FiG. 208. — A fern leaflet, showing the way in which each vein forks into two branches 

 {dicholomous venation), 



inder phloem is present on both sides of the xylem. Other 

 arrangements of the xylem and phloem are found in the 

 stems of other kinds of pteridophytes. 



Reproduction. — On the under side of fern leaves you may 

 have noticed brown dots. (See Figure 209.) You may 

 have noticed that what you now know to be spores come 

 from these dots. These brown dots are groups of spore 

 cases and these spore cases are called sporangia. The 

 brown groups of sporangia are called sori. When young the 

 sorus is usually protected by a covering called an indusium. 



The appearance of one of these sporangia under the 

 microscope suggests a helmet, such as was worn by the 

 armored knights of old. (See Figure 2og, D.) The crest 

 of the helmet is a row of thick-walled cells called the annulus. 

 When the spores are ripe, the annulus is of service in dis- 

 charging them. It behaves as shown in Figure 211. 



You may confuse the word sporangium with the word 

 sporogonium. Both of these structures contain spores. 

 But sporogonia occur only in bryophytes, and they con- 

 stitute the entire sporophyte generation of the plants which 

 produce them. Also, the spore-producing part of a sporo- 



