212 THE ISOETACE/E. 



Only the outer earlier whorls of leaves produce sporan- 

 gia. When the spores are ripe the fertile leaves slowly 

 decay, but the sterile, somewhat rudimentary leaves 

 remain green through the winter. In the terrestrial 

 species these leaves may form somewhat spiny processes. 

 The tips of ordinary leaves usually end in a long taper- 

 ing point, which, with the hollow interior, is probably 

 responsible for the name of quillwort applied to the 

 plants. 



The quillworts are usually about four years old when 

 they begin to bear spores, and, as might be expected, 

 their manner of fruiting shows most plainly tlieir rela- 

 tionship to the fern allies. Their resemblance to the 

 selaginellas is particularly striking, since they bear two 

 kinds of spores. These are produced in the axils of the 

 outer leaves in a hollovved-out portion of the leaf. In 

 each of these hollows is a single oblong sporangium 

 which may occasionally reach a length of more than a 

 quarter of an inch. Over this sporangium the substance 

 of the leaf projects in the form of a thin indusium or 

 velum, in a few cases entirely covering the sporangium, 

 but usually leaving a third or more exposed. The 

 sporangia are said to be one-celled, but they contain 

 partitions that divide them into several incomplete 

 chambers. This feature is also found in fossil lepidoden- 

 drons. Just beyond the sporangium there arises from 

 the inner surface of the leaf a small, thin, triangular flap 

 called the ligule. It is most strongly developed in ter- 

 restrial forms, but its use is unknown. In many species 

 some of the cells in the walls of the sporangium become 

 thickened, giving it a spotted appearance which is some- 

 times considered of diagnostic importance. The spots, 

 however, are not visible to the unaided eye. 



