HIDDEN MARRIAGES 



509 



FIG. 649. PILLWORT. 



One of the sporocarps enlarged. 



fibres without root-hairs; those of Adders-tongue 

 (Ophioglossum vulgatum) put forth adventitious 

 buds. 



As we have seen, in the Polypodiacese each 

 sporange is formed from a single cell of the epi- 

 derm ; but in the Ophioglossacese a single sporange 

 is the product of a group of specialized cells be- 

 neath the epiderm, and is therefore more homolo- 

 gous to an entire sorus in the true ferns. Its walls, 

 several cells thick, are products of the epiderm, 

 and are still furnished with stomates. Their full 

 development occupies a year, and they are then 



globose in shape, have no annulus, and split transversely to liberate the 

 minute squarish spores. Instead of germinating on the surface as do the 

 spores of ferns, these appear to need burial before developing into pro- 

 thallia ; at least, so far as it has been observed, the prothallium is a subter- 

 ranean tuberous body, devoid of chloro- 

 phyll, and bearing archegones and 

 antherids seated in pits or projections of 

 the upper surface the former in Botry- 

 chium, the latter in Ophioglossum. These 

 organs and their contents are very similar 

 to those of ferns. 



The Order Hydropteridese, or Water- 

 ferns, is a small group consisting of few 

 genera and species, with a solitary British 

 representative, the Pillwort (Pilularia 

 globidifera, fig. 650). They are aquatic or 

 semi-aquatic plants, and the spore-cases 

 (sporocarps) are borne at the base of the 

 leaves. The Pillwort, which grows on the 

 margins of lakes and ponds where it is sub- 

 merged in winter and exposed in summer, 

 consists of a creeping stem from the under 

 side of which are produced at intervals small 

 tufts of fibrous roots, and from the upper 

 surface erect, cylindrical, bristle-like bright 

 green leaves, in whose axils are the short- 

 stalked, globular sporocarps. These sporo- 

 carps, which have a hard shell of scleren- 

 chyma, are divided into four compartments, 

 and in each of these, springing from the 

 broadest face of the wall, is a cushion an- 

 alogous to the placenta in the ovary of 



Fio. 650. PILLWORT (Pilularia 

 globulifera). 



Portion of plant showing s 

 the fronc 



icarps at base of 



