PLANTS FOR THE AQUARIUM 89 



leaves are again all submerged and are opposite. They clasp the 

 stem, which thus appears to pass through a single leaf. 



Related to the pondweeds is Zostera, or grass-wrack, our only 

 truly marine flowering plant, though others occar in salt marshes. 

 Zostera is found at and below low tide-mark, and is important 

 because of the number of beautiful animals which find shelter 

 among its grass-like leaves. It grows on slightly sloping sandy 

 or muddy shores, creeping along in the sand by means of its 

 fleshy rootstock, rooting as it goes. It is very remarkable in 

 that the whole process of flowering and seed-setting is conducted 

 under the water, whereas most flowering plants flower and seed 

 above the water (cf. pondweed). It is therefore one of the most 

 fundamentally modified of the aquatic flowering plants. The 

 leaves grow to a length of from one foot upwards, so that the 

 plant is generally too large for an aquarium, but small specimens 

 may be tried as a curiosity. The teacher should not fail to 

 demonstrate the differences between this plant and the much 

 more delicate and simpler sea-grass (Enteromorpha, cf. p. 84), 

 which is an alga. The nerves or vascular bundles of the leaf 

 will be found to be very distinct, and are numerous and parallel, 

 as in grasses. 



All the above are Monocotyledons, and to them may be added 

 Elodea, or Anacharis canadensis, the Canadian waterweed, which 

 since its introduction into this country has flourished in an extra- 

 ordinary fashion, and is now to be found in almost every stream 

 and canal. The plant is wholly submerged, and though normally 

 attached by roots will thrive in the aquarium if a branch be 

 simply thrown into the water. The colour is dark green, the 

 leaves opposite or in whorls of three or four, sessile, oblong, and 

 semi-transparent. The flowers are very insignificant, and only 

 the female plants are known in Britain, so that reproduction is 

 purely vegetative. It chiefly occurs by the breaking off of twigs. 

 Accustomed to a colder climate than ours, the plant makes no 

 special preparation for our winter ; the ends of the branches with 

 their clustered leaves survive the frost even if the rest of the 

 branch dies. 



Leaving the Monocotyledons, we find among the Dicotyledons a 

 purely aquatic family in the Callitriches. The water-starwort 



