THE QUILLWORT 



167 



embryo, which in due course becomes an independent 

 Selaginella plant. 



Isoetacece. — Plants with short, usually unbranched, 

 fleshy stems, on which alternating whorls of fertile and 

 barren, awl-shaped leaves are borne; the leaves are 

 fleshy, and the roots have forked branches. They grow 

 on very damp soil or submerged in water. The family 

 includes the one genus IsoHtes, with about fifty species, 

 which are found in different parts of the world. Three 

 species occur in Britain, and Isoetes 

 lacuslris, one of these three, known 

 as the European Quillwort, or 

 Merlin's Grass, is a good type of 

 the genus. Its stem is tuberous 

 and very short, almost globular in 

 form. The leaves rise from the 

 crown of the stem; they are from 

 2 to 6 inches long, awl-shaped, and F ^. 56.— Akchegonium 



vi,! ,i • i in ,i or Selaginella ready 



dilated at their base. Above the F0K FERTILIZATION . 

 base they are quadrangular. This x 500. 

 curious plant grows in mountain- 

 pools and shallow lakes in North 

 Britain. At a cursory glance it might easily be 

 mistaken for the Shore- Weed, Litorella lacustris, a 

 flowering-plant of the Plantain family, or even for the 

 Water Lobelia, Lobelia dortmanni. In a pit on the inner 

 side of the dilated base of each leaf a single bulky 

 sporangium is placed, and above it is found a mem- 

 branous, triangular ligule. The plant is heterosporous. 

 The numerous microspores are developed in micro- 

 sporangia, which are found on the inner leaves. The 

 megasporangia are on the outer leaves; each one con- 



O, Ovum ; m, mucilage in 

 canal; n, cells of neck. 



