142 MOUNTAIN AND MOORLAND 



the stomata are absent or vestigial except from the 

 upper surface of floating leaves. (7) Most water 

 plants are perennial, for seasonal changes are often 

 but little marked in aquatic conditions. Vegetative 

 propagation becomes more important than seed-form- 

 ing. It is a general fact that great humidity tells 

 against the forming of flowers, while dryness is in its 

 favour. 



It is well worth while to collect some Duckweed 

 and Pondweed and Bladderwort and leave them quietly 

 in a dish, to get a glimpse of the small animals to 

 which they give shelter. There are small, slow-going 

 Water Snails, there are Water Mites of great activity 

 and attractive colours, there are larval insects of 

 many different kinds, such as very tiny Caddis Flies, 

 there are minute Crustaceans or Water Fleas, besides 

 Leeches and Threadworms and Planarian Worms and 

 minute, single-celled creatures like the Bell Animal- 

 cule (Vorticelld), forming a fringe on the water weed 

 and jerking itself up and down on the end of a flexible 

 stalk whiqh coils into a spiral when contracted. But 

 none is more interesting than the freshwater polyp, 

 the green or brown or grey Hydra, to which we must 

 pay some attention. 



The freshwater polyp was first studied by the Abbe 

 Trembley in the eighteenth century, and it was he who 

 called it Hydra after the mythical monster with which 

 Hercules contended. It seemed a strange name to 

 give to a little tubular animal about half an inch long 

 and the thickness of a pin, but the Abbe had dis- 

 covered that if the polyp was cut into pieces each part 

 could grow into a whole, and that if one of the six to 

 ten tentacles surrounding the mouth was cut off, it 



