OHEMmmt AND PlltSlOS OF PLANTS. 89 



law of physics, separates into pure ice-crystals and a denser 

 unfrozen solution. By a greater reduction of temperature 

 more ice-crystals may be separated out, and the remaining 

 solution made denser still. This increasing density tends 

 to retard the formation of ice-crystals, and it is probable 

 that it is only in extremely low temperatures, if at all, that 

 the liquids in the plant are completely solidified. 



182. A plant which has been frozen may survive in many 

 instances if thawed slowly, but if thawed quickly its vitality 

 is generally destroyed. Thus many herbaceous plants will 

 endure quite severe freezing if they are afterward covered 

 so as to secure a slow rise of the temperature, and many 

 bulbs, tubers, and roots will survive the severest winters if 

 covered deeply enough to prevent sudden thawing. Like- 

 wise tuijgid tissues, which are not living, as those of many 

 succulent fruits, are injured or not by freezing, according 

 as the thawing has been rapid or slow. 



183. Light. — ^AU green plants are directly dependent 

 upon light, for it is only in the light that they can manu- 

 facture starch. Without light they would starve just as 

 surely as would animals if deprived of their proper food. 



184. Light does not appear to be essential to plants in 

 any other way than to enable them to make starch; so that 

 those which get their starch from others can live in total 

 darkness. Thus many saprophytes (i.e., plants which live 

 upon dead or decaying vegetable matter) are found in dark 

 cellars, caves, mines, etc., growing to full size and maturing 

 their fruit perfectly. So, too, some parasites (i.e., plants 

 living upon and getting their food from living plants) grow 

 in darkness, feeding upon the inner tissues of their hosts 

 (supporting plants) where little or no light penetrates. 



185. The flowers and fruits of ordinary plants develop 



