Lecture XXL 



171 



are arranged in four rows down the stem. Two rows of small 

 leaves are along the upper or dorsal aspect of the stem and there 

 are two rows of larger leaves, one along each side. It may also be 

 seen, especially in the older parts of the stem where the leaves are 

 more widely spaced, that the leaves occur in pairs on the stem a 

 large lateral leaf and a small dorsal leaf on the other side being 

 exactly opposite and forming a pair. The leaves of adjacent pairs 

 are on alternate rows. Closer examination with a lens shows a 

 minute shell-like scale on their upper surfaces very near the base. 



FIG. 45. Selaginella, leaf, transverse sections, x 300. eo, upper, eu, under 

 epidermis; I, air-spaces; sp, stomata. (From Evans' An Intermediate 

 Textbook of Botany. After Goebel.) 



This inconspicuous organ is called a ligule. It is to all appearance 

 functionless. It is rarely found on the leaves of living plants, but is 

 of interest inasmuch as it characterises the leaves of important groups 

 of fossil plants. In Selaginella it may be regarded as an heirloom, 

 proving the antiquity of its descent. At the bifurcations of the stem 

 sometimes a stiff wiry organ is found directed from the under sur- 

 face of the stem towards the soil. It is a rhizophore. From its 

 slightly bulbous end thread-like roots grow and penetrate into the 

 soil. In some specimens the ends of some of the branches tend to 

 turn upwards and assume a vertical position. The leaves on these 



