64 CHEMISTRY OF PLANT LIFE 



located to the storage organs and accumulated for later use by 

 the same plant (as, for example, in the case of the perennials), or 

 by the next generation of the plant (when the storage is in the 

 endosperm adjoining the embryo of the seed). 



There is no known explanation as to why different species of 

 plants make use of different carbohydrates for these purposes; 

 or why certain species elaborate starch out of the same raw mate- 

 rials from which other species produce sugars, inulin, or glyco- 

 gen, etc. 



In general, starch is the final product of photosynthesis in 

 most green plants; but there are many exceptions to this. The 

 polysaccharides, which are generally insoluble, must be broken 

 down into the simpler soluble sugars before they can be trans- 

 located to other organs of the plant for immediate, or future, use. 

 When they reach the storage organs, they may be recondensed 

 into insoluble polysaccharides, or stored as soluble sugars. Exam- 

 ples of the latter type of storage are, sucrose in beet roots, glucose 

 in onion bulbs, etc. Sometimes, this habit of storage seems to be a 

 species characteristic; as potatoes store starch, while beets, grow- 

 ing in the same soil and under exactly the same environment, store 

 sugar. But in other cases, the nature of the carbohydrate ^tored 

 undoubtedly is correlated with the external temperatures at the 

 time of storage. It has been shown that cold, which tends to 

 physiological dryness, very frequently favors the storage of sugars 

 instead of starches. Thus, in temperate zones, among aquatic, 

 or moisture-loving plants, those species which hibernate during 

 the winter at the bottom of lakes or ponds and are killed by tem- 

 peratures below freezing, store starch and no sugar; while in the 

 same ponds, the species whose storage organs pass the winter above 

 the level of the water and can withstand temperatures as low as 

 7 C. contain sugar during the winter months, even if they con- 

 tain starch during warmer periods. Similarly, sugars often appear 

 in the leaves and stems of conifers during the winter months, only 

 to disappear, or be replaced by starch, when spring approaches. 

 This same phenomenon is noticeable in arctic plants, which gen- 

 erally contain but small proportions of starch and relatively large 

 amounts of sugars. 



Similarly, the phenomenon of the turning sweet of potatoes when 

 exposed to low temperatures has often been noted. The change of 

 the starch in potato tubers to sugar is most rapid at the tempera- 



