Page:Popular Science Monthly Volume 77.djvu/365

Rh condition (?). Lizards and snakes retire to holes in trees, under stones, dead leaves, and many species may congregate together in large numbers. They are in a quiescent and somnolent state, not true hibernation.

Fish of the temperate zone do not fall into a state of complete torpidity, but their vital functions are diminished and they retire to sheltered 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 molluscæ 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 breathing. 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 hibernation. Fresh-water molluscæ go into a state of hibernation in the fall, burying themselves in the mud until spring. It is believed that saltwater molluscæ. 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 larvæ 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 moisture 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 phenomena take place in tropical countries during the hot season, whenever 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.