60 PLANT PHYSIOLOGY 



visible as soon as formed, and it can be identified by the 

 familiar blue-black color imparted to it by iodine. 



In a very considerable number of plants and plant organs, 

 starch is never formed, whether as the immediate product 

 of photosynthesis or as food in store for future use. In 

 these the food remains as sugars in solution, accumulates in 

 visible form as drops of oil, or may be stored as reserve 

 cellulose. It is in general true that in plants and in organs 

 which, for any reason, should be especially light, oil sepa- 

 rates instead of starch : e. g\ in the Diatoms, most of which 

 float either free or attached ; in many of the Phaeophyceae, 

 in the Characese (except the bulbils), and in such light 

 and mechanically weak structures as the cylindrical leaves 

 of onion, etc. The specific gravity of starch is higher, of oil 

 lower, than that of water. Their nutritive values may be, 

 probably are, about equal. Their values as sources of en- 

 ergy through respiration are not equal.* The advantage 

 of oil as reserve food, or as the immediate product of pho- 

 tosynthesis, where lightness is important, is evident. Sugar 

 never crystallizes in the cell, it remains always in solution, 

 osmotically active (though comparatively weak), readily 

 diffusible, and although occurring much more commonly as 

 food immediately available or in transit from one part of 

 the plant to another, it is sometimes the form in which 

 food is accumulated and stored. 



From considerations insufficiently supported by experi- 



* Complete oxidation (perfect respiration) of oil yields half again as 

 many calories per gram-molecule as starch and sugar. See Chemiker 

 Kalender where heat of combustion of vegetable oils is given, about 932 

 Gals while that of starch and sugar is about 709 Cals. 



According to Stahl (Jahrb. f. wiss. Botanik, Bd. 34, p. 565, 1900), 

 constantly submersed Chlorophycea?, which run no risk of drying up 

 during a period of vegetation, form starch, while on the other hand, those 

 alg* subjected to frequent drying up form sugar. Water would evaporate 

 less and less rapidly from a sugary or other solution of increasing density. 

 The rate of evaporation would not be affected in any way by starch or 

 other substances not in solution. Vaucheria and certain other algae, 

 some species of which are subject to drying up, contain oil in quantity. 

 This cannot reduce the loss of water by evaporation. Are these algse 

 therefore less well protected against harm by drying, or is Stahl's con- 

 jecture a mistaken one? 



