CHAPTER XIII. THE PTERIDOPHVTA. 359 



The internal structure of the stems is somewhat peculiar. 

 There is an axial fibro-vascular cylinder enclosed in a well- 

 developed bundle-sheath, and the cylinder itself contains a 

 number of fibro-vascular bundles which lie side by side, and are 

 distinct from each other, or are united in various ways, but 

 always in such a manner that an axial longitudinal section 

 separates them into two symmetrical halves. The bundles, 

 therefore, sometimes have the appearance of radial bundles. At 

 the outer extremities of each of the bundles occur spiral tracheids 

 of small diameter. 



There are two genera, Lycopodium and Phylloglossum. 



In the principal genus, Lycopodium, the sporangia, which are 

 much larger than those of Ferns, occur singly on the upper 

 surface of leaves near their base. In Lycopodium Selago and 

 a few others, they are borne on ordinary leaves, and these not 

 on any special part of the stem, but in most species they occur 

 in terminal spikes composed of somewhat modified leaves. See 



Fig. 5 6 5- 



Phylloglossum is a genus found only in Australia. The plants 

 are of small size, and the stems bear a rosette of elongated 

 leaves. The axis is continued into a slender stalk above, which 

 bears a spike of small leaves in which the sporangia occur. The 

 stems spring from a tuber, and the plants are multiplied vege- 

 tatively by means of adventitious shoots. 



Order B. Psilotaceae. This is a small group composed 

 entirely of exotic shrubby plants of small size. They are dis- 

 tributed into two genera, one, the genus Tmesipteris, confined to 

 Australia, and the other, Psilotum, occurring in the Sandwich 

 Islands, the Molluccas and Madagascar as well. The Psilotums 

 have erect, dichotomously branching, angular stems, with small, 

 simple and distant leaves. They are entirely without true roots, 

 peculiarly modified branches of the stem taking their place 

 functionally. 



The genus Tmesipteris has unbranching or sparingly branch- 

 ing stems, which have broader, better developed and more 

 numerous leaves, but in most other respects resemble the 

 Psilotums. 



A distinctive peculiarity of the order consists in the way the 

 sporangia are borne. They are sunken in the ends of very short 



