184 SYSTEMATIC BOTANY. 



14. Hydrocharidaceae. The Frog's-bit family con- 

 sists of small aquatic plants, with inconspicuous dioecious or 

 polygamous flowers ; stamens three to twelve, ovary inferior, 

 fruit ripening under water. The American Water-weed 

 {Anacharis Canadensis) is a slender herb, with opposite 

 whorled leaves, growing under water; upon its advent 

 into Europe a few years ago, it multiplied so rapidly as to 

 be troublesome to navigation. The Grass- wrack (Zostera 

 marina), found in tidal rivers, is used for packing, cush- 

 ions, etc. The Tape, or Eel-grass {Vallisneria ^iralis), 

 growing at the bottom of the water, has long tape-like 

 leaves and dioecious flowers. The staminate flowers become 

 detached early, floating on the water, where they expand 

 and shed their pollen. The latter finds its way to the 

 pistillate flowers, whose elongated peduncles allow them 

 also to reach the surface of the water. After fertilization, 

 the peduncles again coil and draw the ovary beneath the 

 water, where its development is completed. 



DICOTYLEDONES. 



189. The Dicotyledones are so called because the first 

 leaves (seed-leaves or cotyledons) of the embryo, or plantlet, 

 are two and opposite (Fig. 277, cot) ; this sub-ckss is also 

 called Exogense, or Exogenous plants, because the 

 growth or increase of the wood each year (in case of the 

 perennial woody species) is in the form of a concentric ring, 

 or cylinder, external to that previously formed. A trans- 

 verse section of a very young plant will show the presence 

 of a few open fibro-vascular bundles, arranged in the form 

 of a circle (Fig. 295). Other bundles soon arise between 

 these first formed (Fig. 296), and finally become so numer- 

 ous as to lie against each other, and form a cylinder, the 



