88 LIVING PLANTS 



Ice in the 

 tissues 



remainder of the jjlant is frozen solidly. With- 

 out doubt other differences also exist. 



To understand these appearances one must 

 recall the salient features in the structure of 

 the leaf: that it is composed of a mass 

 of looselj'-arranged, thin-walled, globular, 

 cylindrical or irregular sacs lined with pro- 

 toplasm and containing seventy to ninety 

 per cent of their volume of water. The loosely- 

 arranged cells are held in position by the 

 strong mechanical tissue of the ribs or nerves, 

 and the whole enclosed by the single layer of 

 epidermal cells : that of the lower side has 

 openings (stomata) which permit the escape 

 of watery vapor accumulating in the spaces 

 among the inner cells. 



If now a section is made of a frozen leaf, it 

 will be found that the spaces between the cells 

 usually containing air are filled almost solidly 

 wnth ice crystals. From whence is this ice de- 

 rived ? It will be remembered that the cell 

 contains a large proportion of water, some 

 of which is in the form of a solution of acids, 

 salts, etc., in the cavities of the cell, and some 

 in the form of water of imbibition in the pro- 

 toplasm and in the cell wall. The water of 

 imbibition may be imagined as filling up the 

 minute spaces between the groups of mole- 

 cules in the cell wall and the protoplasm. Now 

 it is a well known principle in physics that 



