PTERIDOPHYTA 



223 



parenchyma with a small amount of xylem in its upper portion. Appar- 

 ently no phloem is present. In sterile plants the vascular system is a 

 protostele, but in fertile plants the .xylem surrounds a central mass of 

 parenchyma and is thus a siphonostele. The development of the xylem 



Fig. 179. Lycopodium rejlcxum, an upright tropical .species in which most of the leaves are 

 sporophylis; about natural size. 



is mesarch, the metaxylem arising both inside and outside the protoxylem. 

 This is a more advanced condition than that seen in Lycopodium. 



Sporangium. The sporangia of Lycopodium are borne singly in the leaf 

 axils. Each sporangium is large and more or less kidney-shaped, with a 

 very short stalk, a wall several layers of cells in thickness, and a central 

 mass of small, yellow, thick-walled spores. When mature, it dehisces by 

 means of a transverse slit. A leaf that bears a sporangium is termed a 

 sporophyll. 



In the simplest species of Lycopodium every leaf on the plant is a sporo- 



