CHEMISTRY AND PHYSICS 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 turgid 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.—All 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 
