64 LIFE : OUTLINES OF GENERAL BIOLOGY 



V. Hibernation, as in hedgehog, dormouse, marmot, and some 

 bats, where there are profound changes in the metaboHsm 

 of the body, and a relapse from the characteristic mam- 

 malian warm-bloodedness. Here should be included the 

 similar state of astivation, which occurs as a reaction to 

 extreme heat. 

 VL By themselves, and probably to be regarded as regularised 

 pathological conditions are the de-dillercntiations of some 

 Pol}'zoa and Tunicates, where the bod}- or part of it under- 

 goes collapse and passes into a resting stage with much 

 simplified structure, from which, when prosperous conditions 

 return, there may be an emergence of a rejuvenated and 

 reconstructed organism with a new lease of lile. 



IN ILLUSTRATION : FROZEN PLANTS. Living matter usually 

 contains about 70 per cent, of water, and there can be no under- 

 standing of vital processes without recognising the r61e of water. 

 On the other hand, in times of prolonged frost we cannot but think 

 of the danger that organisms run in having so much water in their 

 composition. Why does this water not freeze? and if it freezes will 

 this not mean death? In many places there is often lasting frost 

 such as Shelley pictured: 



A winter such as when birds die 

 In the deep forests, and the fishes He 

 Stiffened in the translucent ice, which makes 

 Even the mud and slime of the warm lakes 

 A wrinkled clod, as hard as brick. 



Our question is how tlie plants get on during a i)rolunged, severe 

 frost. We do not refer to those that die down so that they are wholly 

 protected by the earth, and it may be a blanket of snow; nor to 

 those that are specially protected by thick epidermis, or by wool, 

 or by resinous bud-scales, or by bark, and so on; we are thinking 

 of ordinary plants like daisies and groundsel, and the glossy ivy, 

 and the grass. Mow do they survive when the temperature falls 

 below zero? What charm have they that many imported garden 

 I)lants have not ? 



Those who grow plants in an unheated greenhouse are familiar 

 with the wilting of some kinds of leaves in very cold weather. 

 Ex|HTiments show that this is because the leaves are continuing to 

 give off water vapour while the roots in the cold earth are unable 

 to absorb enough to make good the loss. Thus the leaves sink down 

 wilted and may eventually die, though there is no actual freezing. 



Hut Prof. Molisch has shown that in ca.ses where the loss of 

 water and warmth from the leaves is checked by using a bell-jar 

 or by some other device, there may be a dying away, although the 



