HIBERNATION 359 



condition ( ?) . Lizards and snakes retire to holes in trees, under stones, 

 dead leaves, and many species may congregate together in large num- 

 bers. They are in a quiescent and somnolent state, not true hibernation. 

 Fish of the temperate zone do not fall into a state of complete tor- 

 pidity, but their vital functions are diminished and they retire to shel- 

 tered holes and cease to go abroad in search of food. In the tropics 

 (Africa and India) large numbers of fish are known to survive long 

 and severe droughts during which the streams and ponds are completely 

 dried up. This, they do, by passing the dry season embedded in 

 the mud. 



Most of the species of molluscse hibernate. The land snails bury 

 themselves in the ground or conceal themselves under the bark of trees ; 

 in fact in almost any sort of a cavity to be found. They close the 

 mouth of their shells with a calcareous plate technically known as the 

 " epiphragm," and this is perforated by a minute hole to permit breath- 

 ing. The substance forming this plate they secrete in their mouths. 

 During the dry weather in summer, the snails bury themselves in the 

 ground and cover the opening of their shells with this protective shield, 

 but it is much thinner than the one used in winter. This they do to 

 protect themselves from the drought, i. e., by checking evaporation. 

 Slugs bury themselves but do not enter into a complete state of hiber- 

 nation. Fresh-water molluscse go into a state of hibernation in the fall, 

 burying themselves in the mud until spring. It is believed that salt- 

 water mollusca?. hibernate in a similar manner, but practically nothing 

 is known concerning them. 



Many butterflies and moths hibernate in the perfect state as well 

 as in the form of imagos, but not in the larval state ( ?). Most insects 

 which pass the winter in a state of larvse hibernate during the period 

 when they can not obtain any food. Insects which hibernate do not 

 pair until spring and bees do not hibernate at all. It is well known to 

 bee-keepers that these insects need plenty of food during the winter 

 months. 



In the seeds of plants and in the eggs of many of the lower animals, 

 life may remain dormant for years in cold climates, until heat or mois- 

 ture awakens them. Many plants die down, while their roots remain 

 alive during the winter season (perennials and biennials), coming to 

 foliage and blossom in the spring. In the same way trees shed their 

 leaves in the autumn and the sap returns to the roots. Similar phe- 

 nomena take place in tropical countries during the hot season, when- 

 ever the amount of humidity in the atmosphere is sufficient to maintain 

 perennial vegetation during the entire year. 



These phenomena in the vegetable world are regarded generally as 

 being analogous to those of hibernation in animals and therefore the 

 terms " hibernation of plants " is sometimes applied to them. 



