86 THE BOOK OF NATURE STUDY 



when they develop into new plants. These interesting plants 

 can be kept in an aquarium if the base is carefully inserted in 

 sand or mud. The coated egg-cell is interesting, because it is 

 common for both plants and animals in fresh water to pass the 

 winter within a protective egg-case. In this way they escape 

 the effects of frost, which may destroy the parents completely. 



2. Aquatic Flowering Plants. In marked contrast to the algae 

 the flowering plants to be found in ponds and streams have had 

 a terrestrial ancestry. Whereas the algae in their structure and 

 methods of reproduction show that they have been descended 

 from ancestors which always lived in water, the flowering plants 

 show, both as regards structure and methods of reproduction, 

 peculiarities which we can only explain on the hypothesis that 

 their ancestors lived on dry land. Very little observation in the 

 field will show further that there is an almost continuous tran- 

 sition between plants which can only live in wet places, through 

 marsh plants, to those forms which are permanently submerged. 

 It is only the latter with which the aquarium-keeper as such is, 

 strictly speaking, concerned, for it is only plants some at least of 

 whose leaves are submerged that will generate the desired bubbles 

 of oxygen. In passing, however, one should notice the existence 

 of such interesting marsh plants as pinguicula, sundew, red rattle, 

 bog pimpernel, and so forth, which though they do not grow 

 beneath the surface of the water, yet can only thrive in its 

 vicinity. The next stage, as it were, is found in the duckweed 

 (Lemna), which often forms a nearly continuous covering over 

 ponds. Lemna is of no direct use as an oxygen-generator, for 

 its starch-making organs float on the surface, but it is a useful 

 aquarium plant none the less, for it affords food and shelter to 

 not a few animals, and it also forms a covering over the surface 

 of the water which gives a grateful shade to the animals at the 

 bottom. Duckweed is a very simple plant, consisting of small 

 leaf-like " fronds," really the stems, which float at the surface, 

 and have slender roots hanging down from their under-surface 

 into the water. They multiply by budding, or, more rarely, by 

 very simple flowers. In autumn buds arise which sink to the 

 bottom of the pond and start growth again in the spring. The 



