4 8 



THE BOOK OF NATURE STUDY 



creeping, with flowers in slender spikes, and the perianth of the 

 flowers dotted with small glands. It is found in wet ditches and 

 on the edges of ponds and streams. 



It seems probable that aquatic plants have originated from 

 land plants, which, crowded out by competition with each other, 

 may have lived an amphibious life for a time, and eventually 

 taken refuge in the water altogether. Now they are so perfectly 

 adapted to life in the water that they would find it difficult to 

 live on land ; it is easier for land plants to adapt themselves 



to an aquatic existence 

 than for water plants to 

 return to a mode of life 

 determined by atmos- 

 pheric conditions. The 

 influence of change in 

 external conditions on 

 land plants has been in- 

 vestigated in the Cuckoo 

 flower (Cardamine pra- 

 tensis), plants of which 

 were found submerged 

 on the banks of a pond 

 that had overflowed. 

 These submerged forms 

 differed from the terres- 

 trial forms in the follow- 

 ing respects : the cauline 

 leaves, which are usually 

 sessile, had developed 

 long stalks ; their segments were narrower ; the epidermis 

 thinner. The nbro-vascular bundles of the stem were nearer 

 the centre, and the cortex was much thicker. Some of these 

 changes would take place if the plant were grown in a medium 

 of moist air instead of water; the two changes which appear 

 to be specially associated with the liquid condition of the water 

 are the displacement of the vascular bundles and the narrow- 

 ing of the leaf segments. Similar results have been observed in 

 one of the Water Crowfoots, Ranunculus fluitans, and in other 



FlG. 1 8. Water Crowfoot (Ranunculus aquatilis). 



