FREEZING AND BURNING. 



551 



steinia geoides) maintain themselves beneath it, unfrozen, even in very severe 

 winters. 



Other plants, again, appear to be protected against extreme cold by the fact 

 that they retire underground during the winter. Large numbers of bulbous and 

 tuberous plants manufacture organic compounds in their green leaves in the warm 

 sunbeams of summer, at once transmitting them below to their subterranean 

 portions. There, thick stems and tubers, fleshy scale-like leaves, and the rudi- 

 ments of new foliage and flowers (which, however, do not appear above-ground 



Fig. 136. — Detachment of special shoots of Potamogeton crispus, for hibernation under water. 



the same year) are produced from the materials supplied. Throughout the winter 

 these structures remain buried in the earth, and are there protected against 

 excessive cold, just like roots. After the winter is over, the flower-stalks and 

 foliage-leaves, commenced in the previous year, rise up in order to bloom and 

 fructify, and to form anew, in the sunlight, organic materials for the subterranean 

 bulbs, tubers, and root-stocks. It is very interesting to notice that bulbs and tubers 

 bury themselves deeper in the earth the more exposed their habitat to radiation 

 and cooling, the more they are threatened with the danger that the earth will be 

 covered by only a thin mantle of snow. While, for example, the bulbs and tubers of 

 Gagea lutea and Corydalis cava, when growing in the black humus of beech forests 

 under withered foliage, lie only a few centimetres below the surface, in open 



